fix: merge account balances preserve fields

packages/assets-controller/CHANGELOG.md

@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Fixed
+
+- Preserve existing per-asset balance fields when applying merge-mode balance updates, so amount-only updates do not drop extra fields already present on the prior balance ([#9453](https://github.com/MetaMask/core/pull/9453))
+
 ## [10.2.0]
 
 ### Added

packages/assets-controller/src/AssetsController.test.ts

@@ -1807,6 +1807,40 @@ describe('AssetsController', () => {
       });
     });
 
+    it('preserves extra balance fields when merge update changes amount', async () => {
+      const initialState: Partial<AssetsControllerState> = {
+        assetsBalance: {
+          [MOCK_ACCOUNT_ID]: {
+            [MOCK_ASSET_ID]: {
+              amount: '1',
+              metadata: { limit: '1000', authorized: true },
+            },
+          },
+        },
+      };
+
+      await withController({ state: initialState }, async ({ controller }) => {
+        await controller.handleAssetsUpdate(
+          {
+            updateMode: 'merge',
+            assetsBalance: {
+              [MOCK_ACCOUNT_ID]: {
+                [MOCK_ASSET_ID]: { amount: '2' },
+              },
+            },
+          },
+          'TestSource',
+        );
+
+        expect(
+          controller.state.assetsBalance[MOCK_ACCOUNT_ID]?.[MOCK_ASSET_ID],
+        ).toStrictEqual({
+          amount: '2',
+          metadata: { limit: '1000', authorized: true },
+        });
+      });
+    });
+
     it('preserves existing balances when merge update adds new chain data', async () => {
       const polygonNative = 'eip155:137/slip44:966' as Caip19AssetId;
       const initialState: Partial<AssetsControllerState> = {

packages/assets-controller/src/AssetsController.ts

@@ -582,7 +582,16 @@ function mergeAccountBalances(
   replaceCoveredChains: boolean,
 ): Record<string, AssetBalance> {
   if (!replaceCoveredChains) {
-    return { ...previousBalances, ...accountBalances };
+    // Per-asset shallow merge so amount updates do not drop extra fields
+    // already on the prior balance (e.g. enrichment metadata).
+    const next: Record<string, AssetBalance> = { ...previousBalances };
+    for (const [assetId, balance] of Object.entries(accountBalances)) {
+      next[assetId] = {
+        ...(previousBalances[assetId] ?? { amount: '0' }),
+        ...balance,
+      };
+    }
+    return next;
   }
 
   const coveredChains = new Set(