diff --git a/src/Model/Model_Required_Fonts.php b/src/Model/Model_Required_Fonts.php
index 052ee3517..dc46e9754 100644
--- a/src/Model/Model_Required_Fonts.php
+++ b/src/Model/Model_Required_Fonts.php
@@ -122,7 +122,8 @@ public function flush(): void {
 	}
 
 	/**
-	 * A font key's files on the core manifest (none for bundled and custom fonts)
+	 * A font key's files on the core manifest (none for bundled and custom fonts), with those of its `sip-ext` font,
+	 * which mPDF loads with it for the SIP characters (Sun-ExtA's Sun-ExtB)
 	 *
 	 * @param mixed $key A font key
 	 *
@@ -131,22 +132,55 @@ public function flush(): void {
 	 * @since 6.19.0
 	 */
 	public function get_font_files( $key ): array {
-		$family = $this->font_sources->get_font_data()[ $this->normalise_font( $key ) ] ?? null;
+		$fontdata = $this->font_sources->get_font_data();
+		$family   = $fontdata[ $this->normalise_font( $key ) ] ?? null;
 		if ( ! is_array( $family ) ) {
 			return [];
 		}
 
+		/* One level only, as mPDF doesn't follow a sip-ext font's own */
+		$families = [ $family, $fontdata[ $this->get_sip_ext( $family ) ] ?? null ];
+
 		$files = [];
-		foreach ( [ 'R', 'B', 'I', 'BI' ] as $style ) {
-			$file = is_string( $family[ $style ] ?? null ) ? $this->font_sources->get_manifest_file( $family[ $style ] ) : '';
-			if ( $this->font_sources->is_core_font_file( $file ) ) {
-				$files[] = $file;
+		foreach ( array_filter( $families, 'is_array' ) as $linked ) {
+			foreach ( [ 'R', 'B', 'I', 'BI' ] as $style ) {
+				$file = is_string( $linked[ $style ] ?? null ) ? $this->font_sources->get_manifest_file( $linked[ $style ] ) : '';
+				if ( $this->font_sources->is_core_font_file( $file ) ) {
+					$files[] = $file;
+				}
 			}
 		}
 
 		return array_values( array_unique( $files ) );
 	}
 
+	/**
+	 * The font keys whose `sip-ext` font is the key, so get_font_files() counts the key's files as theirs
+	 *
+	 * @return string[]
+	 *
+	 * @since 6.19.0
+	 */
+	protected function get_sip_parents( string $key ): array {
+		$parents = [];
+		foreach ( $this->font_sources->get_font_data() as $parent => $family ) {
+			if ( is_array( $family ) && $this->get_sip_ext( $family ) === $key ) {
+				$parents[] = (string) $parent;
+			}
+		}
+
+		return $parents;
+	}
+
+	/**
+	 * The font key mPDF loads with a family for its SIP characters, or an empty string
+	 *
+	 * @since 6.19.0
+	 */
+	protected function get_sip_ext( array $family ): string {
+		return $this->normalise_font( $family['sip-ext'] ?? '' );
+	}
+
 	/**
 	 * Files that should be on disk: default, language, required-source and PDF-selected fonts
 	 *
@@ -237,13 +271,16 @@ static function ( array $reason ) use ( $none ): array {
 	 * @since 6.19.0
 	 */
 	public function is_font_in_use( string $key, bool $network = false ): bool {
-		if ( isset( $this->get_settings_font_reasons()[ $key ] ) ) {
+		/* A font whose sip-ext font this is needs its files too */
+		$keys = array_merge( [ $key ], $this->get_sip_parents( $key ) );
+
+		if ( array_intersect_key( $this->get_settings_font_reasons(), array_flip( $keys ) ) !== [] ) {
 			return true;
 		}
 
 		$this->form_read_failed = false;
 		foreach ( $this->each_pdf_font() as $font => $unused ) {
-			if ( $font === $key ) {
+			if ( in_array( $font, $keys, true ) ) {
 				return true;
 			}
 		}
@@ -259,7 +296,7 @@ public function is_font_in_use( string $key, bool $network = false ): bool {
 
 		$sites = $this->get_site_font_records();
 		foreach ( $this->get_other_site_ids() as $site_id ) {
-			if ( ! isset( $sites[ $site_id ] ) || in_array( $key, $sites[ $site_id ], true ) ) {
+			if ( ! isset( $sites[ $site_id ] ) || array_intersect( $keys, $sites[ $site_id ] ) !== [] ) {
 				return true;
 			}
 		}
diff --git a/tests/phpunit/integration/Model/Test_Model_Font_Downloader.php b/tests/phpunit/integration/Model/Test_Model_Font_Downloader.php
index 4a8c2b0ed..274fa9299 100644
--- a/tests/phpunit/integration/Model/Test_Model_Font_Downloader.php
+++ b/tests/phpunit/integration/Model/Test_Model_Font_Downloader.php
@@ -972,6 +972,16 @@ public function test_prune_replaces_needed_installer_sun_ext_fonts() {
 		$this->assertSame( [ [] ], $this->get_scheduled_args() );
 	}
 
+	public function test_prune_replaces_an_installer_sun_extb_a_sun_exta_default_font_needs() {
+		GPDFAPI::get_options_class()->update_option( 'default_font', 'sun-exta' );
+		wp_clear_scheduled_hook( Model_Font_Downloader::SYNC_EVENT );
+		$this->set_installer_manifest( [ 'Sun-ExtB.ttf' => 'Chewy.ttf' ] );
+		$this->copy_fixture( 'Chewy.ttf', 'Sun-ExtB.ttf' );
+
+		$this->assertSame( [ 'Sun-ExtB.ttf' ], $this->downloader->prune() );
+		$this->assertSame( [ [] ], $this->get_scheduled_args() );
+	}
+
 	public function test_the_replacement_for_an_installer_garuda_lands_under_its_plain_name() {
 		$this->use_shipped_installer_manifest();
 		$this->serve_fixture_as( 'Garuda-v1.1.1.ttf', 'Garuda.ttf' );
diff --git a/tests/phpunit/integration/Model/Test_Model_Required_Fonts.php b/tests/phpunit/integration/Model/Test_Model_Required_Fonts.php
index c9467616d..1beb6da1d 100644
--- a/tests/phpunit/integration/Model/Test_Model_Required_Fonts.php
+++ b/tests/phpunit/integration/Model/Test_Model_Required_Fonts.php
@@ -59,7 +59,8 @@ public function test_it_is_registered() {
 
 	public function test_get_font_files_lists_a_core_keys_files_on_the_manifest() {
 		$this->assertSame( [ 'DejaVuSerif.ttf', 'DejaVuSerif-Bold.ttf', 'DejaVuSerif-Italic.ttf', 'DejaVuSerif-BoldItalic.ttf' ], $this->required->get_font_files( ' DejaVuSerif ' ) );
-		$this->assertSame( [ 'Sun-ExtA.ttf' ], $this->required->get_font_files( 'sun-exta' ) );
+		$this->assertSame( [ 'Sun-ExtA.ttf', 'Sun-ExtB.ttf' ], $this->required->get_font_files( 'sun-exta' ) );
+		$this->assertSame( [ 'Sun-ExtB.ttf' ], $this->required->get_font_files( 'sun-extb' ) );
 		$this->assertSame( [], $this->required->get_font_files( 'arimo' ) );
 		$this->assertSame( [], $this->required->get_font_files( 'notafont' ) );
 	}
@@ -71,6 +72,47 @@ public function test_get_font_files_skips_a_custom_font() {
 		$this->assertSame( [], $this->required->get_font_files( 'chewy' ) );
 	}
 
+	public function test_get_font_files_counts_a_sip_ext_font_one_level_deep() {
+		add_filter(
+			'mpdf_font_data',
+			static function ( array $fontdata ): array {
+				$fontdata['dejavuserif']['sip-ext']   = ' FreeSerif ';
+				$fontdata['freeserif']['sip-ext']     = 'dejavuserif';
+				$fontdata['garuda']['sip-ext']        = 'notafont';
+				$fontdata['kaputaunicode']['sip-ext'] = [ 'freeserif' ];
+
+				return $fontdata;
+			},
+			PHP_INT_MAX
+		);
+
+		$required = $this->required_with( $this->stub_font_sources() );
+		$dejavu   = [ 'DejaVuSerif.ttf', 'DejaVuSerif-Bold.ttf', 'DejaVuSerif-Italic.ttf', 'DejaVuSerif-BoldItalic.ttf' ];
+
+		$this->assertSame( array_merge( $dejavu, self::$freeserif ), $required->get_font_files( 'dejavuserif' ) );
+		$this->assertSame( array_merge( self::$freeserif, $dejavu ), $required->get_font_files( 'freeserif' ) );
+		$this->assertSame( self::GARUDA, $required->get_font_files( 'garuda' ) );
+		$this->assertSame( [ 'kaputaunicode.ttf' ], $required->get_font_files( 'kaputaunicode' ) );
+	}
+
+	public function test_a_sun_exta_default_font_requires_sun_extb() {
+		GPDFAPI::get_options_class()->update_option( 'default_font', 'sun-exta' );
+
+		$this->assertSame( [ 'Sun-ExtA.ttf', 'Sun-ExtB.ttf' ], $this->required->get_required_font_files() );
+		$this->assertSame( [ 'Sun-ExtA.ttf', 'Sun-ExtB.ttf' ], $this->required->get_install_font_files( [ 'sun-exta' ] ) );
+	}
+
+	public function test_a_sip_ext_font_is_in_use_while_a_font_linking_it_is() {
+		$this->assertFalse( $this->required->is_font_in_use( 'sun-extb' ) );
+
+		GPDFAPI::get_options_class()->update_option( 'default_font', 'sun-exta' );
+		$this->assertTrue( $this->required->is_font_in_use( 'sun-extb' ) );
+
+		GPDFAPI::get_options_class()->delete_option( 'default_font' );
+		$this->add_form_with_pdfs( [ [ 'font' => 'sun-exta' ] ] );
+		$this->assertTrue( $this->required->is_font_in_use( 'sun-extb' ) );
+	}
+
 	/**
 	 * @dataProvider provider_junk_font_data
 	 *
@@ -1003,6 +1045,19 @@ public function test_a_site_without_a_usable_record_counts_as_using_every_core_f
 		$this->assertTrue( $this->required->is_font_in_use( 'dejavuserif', true ) );
 	}
 
+	public function test_a_sip_ext_font_is_in_use_while_another_site_uses_a_font_linking_it() {
+		if ( ! is_multisite() ) {
+			$this->markTestSkipped( 'Multisite only' );
+		}
+
+		$site = self::factory()->blog->create();
+		update_network_option( get_main_network_id(), Model_Required_Fonts::SITE_FONTS_PREFIX . $site, [ 'dejavuserif' ] );
+		$this->assertFalse( $this->required->is_font_in_use( 'sun-extb', true ) );
+
+		update_network_option( get_main_network_id(), Model_Required_Fonts::SITE_FONTS_PREFIX . $site, [ 'sun-exta' ] );
+		$this->assertTrue( $this->required->is_font_in_use( 'sun-extb', true ) );
+	}
+
 	public function test_a_failed_form_read_records_nothing() {
 		if ( ! is_multisite() ) {
 			$this->markTestSkipped( 'Multisite only' );