Take a copy before prefilling, as it may be a frozendict

synapse/storage/state.py

@@ -235,7 +235,7 @@ def _store_mult_state_groups_txn(self, txn, events_and_contexts):
                 self._state_group_cache.update,
                 self._state_group_cache.sequence,
                 key=context.state_group,
-                value=context.current_state_ids,
+                value=dict(context.current_state_ids),
                 full=True,
             )