Fixed #33322 -- Fixed loss of assigned related object when saving relation with bulk_update().

django/db/models/query.py

@@ -549,6 +549,8 @@ class QuerySet:
             raise ValueError('bulk_update() cannot be used with primary key fields.')
         if not objs:
             return 0
+        for obj in objs:
+            obj._prepare_related_fields_for_save(operation_name='bulk_update', fields=fields)
         # PK is used twice in the resulting update query, once in the filter
         # and once in the WHEN. Each field will also have one CAST.
         connection = connections[self.db]