And the backend

src/documents/serialisers.py

@@ -1220,11 +1220,19 @@ class DocumentSerializer(
             prev_tags = set(instance.tags.all())
             requested_tags = set(validated_data["tags"])
 
-            # Tags being removed in this update and all descendants
+            # Tags newly added in this update and the ancestors they require
+            added_tags = requested_tags - prev_tags
+            required_by_add_tags = set(added_tags)
+            for t in added_tags:
+                required_by_add_tags.update(t.get_ancestors())
+
+            # Tags being removed in this update and all descendants, except
+            # those required by a tag that is being added in this same update
             removed_tags = prev_tags - requested_tags
             blocked_tags = set(removed_tags)
             for t in removed_tags:
                 blocked_tags.update(t.get_descendants())
+            blocked_tags.difference_update(required_by_add_tags)
 
             # Add all parent tags
             final_tags = set(requested_tags)

src/documents/tests/test_tag_hierarchy.py

@@ -75,6 +75,19 @@ class TestTagHierarchy(DirectoriesMixin, APITestCase):
         tags = set(self.document.tags.values_list("pk", flat=True))
         assert tags == {self.parent.pk, self.child.pk}
 
+    def test_document_api_add_child_keeps_parent_already_assigned(self) -> None:
+        # https://github.com/paperless-ngx/paperless-ngx/issues/13970
+        inbox = Tag.objects.create(name="Inbox", is_inbox_tag=True)
+        self.document.add_nested_tags([inbox, self.parent])
+        self.client.patch(
+            f"/api/documents/{self.document.pk}/",
+            {"tags": [self.child.pk]},
+            format="json",
+        )
+        self.document.refresh_from_db()
+        tags = set(self.document.tags.values_list("pk", flat=True))
+        assert tags == {self.parent.pk, self.child.pk}
+
     def test_document_api_remove_parent_removes_children(self) -> None:
         self.document.add_nested_tags([self.parent, self.child])
         self.client.patch(