diff --git a/packages/host/app/services/store.ts b/packages/host/app/services/store.ts
index 6a649e8f23..9b30e9e03a 100644
--- a/packages/host/app/services/store.ts
+++ b/packages/host/app/services/store.ts
@@ -15,7 +15,7 @@ import { task } from 'ember-concurrency';
 
 import { cloneDeep } from 'lodash-es';
 import { isEqual } from 'lodash-es';
-import { merge } from 'lodash-es';
+import { merge, mergeWith } from 'lodash-es';
 
 import { TrackedObject, TrackedMap } from 'tracked-built-ins';
 
@@ -926,7 +926,11 @@ export default class StoreService extends Service implements StoreInterface {
       omitQueryFields: true,
     });
     if (patch.attributes) {
-      doc.data.attributes = merge(doc.data.attributes, patch.attributes);
+      doc.data.attributes = mergeWith(
+        doc.data.attributes,
+        patch.attributes,
+        (_dest, src) => (Array.isArray(src) ? src : undefined),
+      );
     }
     if (patch.relationships) {
       let mergedRel = mergeRelationships(
diff --git a/packages/host/tests/integration/commands/patch-instance-test.gts b/packages/host/tests/integration/commands/patch-instance-test.gts
index b52a7abea4..cef5e90815 100644
--- a/packages/host/tests/integration/commands/patch-instance-test.gts
+++ b/packages/host/tests/integration/commands/patch-instance-test.gts
@@ -215,6 +215,54 @@ module('Integration | commands | patch-instance', function (hooks) {
     );
   });
 
+  test<TestContextWithSave>('patching a containsMany field with a shorter array fully replaces it (no stale trailing items)', async function (assert) {
+    let patchInstanceCommand = new PatchCardInstanceCommand(
+      commandService.commandContext,
+      {
+        cardType: PersonDef,
+      },
+    );
+    let url = new URL(`${testRealmURL}Person/hassan`);
+    let saves = 0;
+    this.onSave((saveURL) => {
+      if (saveURL.href === url.href) {
+        saves++;
+      }
+    });
+
+    await patchInstanceCommand.execute({
+      cardId: `${testRealmURL}Person/hassan`,
+      patch: { attributes: { nickNames: ['Paper', 'Pinky', 'Pix'] } },
+    });
+    await waitUntil(() => saves > 0, {
+      timeout: saveWaitTimeoutMs,
+      timeoutMessage: 'timed out waiting for the first save',
+    });
+
+    let savesAfterGrow = saves;
+    await patchInstanceCommand.execute({
+      cardId: `${testRealmURL}Person/hassan`,
+      patch: { attributes: { nickNames: ['Paper'] } },
+    });
+    await waitUntil(() => saves > savesAfterGrow, {
+      timeout: saveWaitTimeoutMs,
+      timeoutMessage: 'timed out waiting for the second save',
+    });
+
+    let result = await indexQuery.instance(url);
+    let instance =
+      result && result.type === 'instance' ? result.instance : undefined;
+    assert.ok(instance, 'instance payload is present');
+    if (!instance) {
+      throw new Error('expected instance payload');
+    }
+    assert.deepEqual(
+      instance.attributes?.nickNames,
+      ['Paper'],
+      'the containsMany array was fully replaced, not index-merged',
+    );
+  });
+
   test<TestContextWithSave>('can patch a linksTo field', async function (assert) {
     let patchInstanceCommand = new PatchCardInstanceCommand(
       commandService.commandContext,