flatten values already corrupted in db at display time

Author: TarekRemo

inc/field.class.php

@@ -1283,6 +1283,18 @@ public static function prepareHtmlFields(
                 $value = is_array($decoded) ? $decoded : [];
             }
 
+            if ($field['multiple'] && is_array($value)) {
+                // Flatten any nested arrays caused by corrupted DB data (double-encoded values)
+                // so that Dropdown::show() always receives a flat list of scalars.
+                $value = array_values(array_filter(
+                    array_merge(...array_map(
+                        static fn($v) => is_array($v) ? array_values($v) : [$v],
+                        $value,
+                    )),
+                    is_scalar(...),
+                ));
+            }
+
             $field['value'] = $value;
         }