diff --git a/synapse/api/room_versions.py b/synapse/api/room_versions.py
index fe4f97d128..3e06819424 100644
--- a/synapse/api/room_versions.py
+++ b/synapse/api/room_versions.py
@@ -36,7 +36,7 @@ class EventFormatVersions:
     ROOM_V3 = 2  # MSC1659-style $hash event id format: used for room v3
     ROOM_V4_PLUS = 3  # MSC1884-style $hash format: introduced for room v4
     ROOM_V11_HYDRA_PLUS = 4  # MSC4291 room IDs as hashes: introduced for room HydraV11
-    ROOM_VMSC4242 = 5 # MSC4242 state dags: adds prev_state_events, removes auth_events
+    ROOM_VMSC4242 = 5  # MSC4242 state dags: adds prev_state_events, removes auth_events
 
 
 KNOWN_EVENT_FORMAT_VERSIONS = {
diff --git a/synapse/storage/controllers/persist_events.py b/synapse/storage/controllers/persist_events.py
index de9487e3dd..b75ce107c4 100644
--- a/synapse/storage/controllers/persist_events.py
+++ b/synapse/storage/controllers/persist_events.py
@@ -635,11 +635,8 @@ class EventsPersistenceStorageController:
 
         # Get the room version for the first event. This room version is the same for all events
         # as events_and_contexts is all for one room.
-        room_version = (
-            events_and_contexts[0][0].room_version
-            if len(events_and_contexts) > 0
-            else None
-        )
+        assert len(events_and_contexts) > 0
+        room_version = events_and_contexts[0][0].room_version
 
         for chunk in chunks:
             # We can't easily parallelize these since different chunks
@@ -656,17 +653,16 @@ class EventsPersistenceStorageController:
                         name="_process_state_dag_forward_extremities_and_state_delta",
                         server_name=self.server_name,
                     ):
+                        assert all(
+                            isinstance(ev, FrozenEventVMSC4242) for ev, _ in chunk
+                        )
                         (
                             new_forward_extremities,  # for prev_events
                             state_delta_for_room,  # for state groups
                             new_state_dag_extrems,  # for prev_state_events
                         ) = await self._process_state_dag_forward_extremities_and_state_delta(
                             room_id,
-                            [
-                                (ev, ctx)
-                                for ev, ctx in chunk
-                                if isinstance(ev, FrozenEventVMSC4242)
-                            ],
+                            cast(list[tuple[FrozenEventVMSC4242, EventContext]], chunk),
                         )
                 else:
                     with Measure(
@@ -864,9 +860,6 @@ class EventsPersistenceStorageController:
             existing_state_dag_fwd_extrems,
             event_contexts,
         )
-        assert new_state_dag_fwd_extrems, (
-            f"No state dag forward extremities left in room {room_id}!"
-        )
         # ...and the room DAG
         existing_room_dag_fwd_extrems = (
             await self.main_store.get_latest_event_ids_in_room(room_id)
@@ -896,6 +889,7 @@ class EventsPersistenceStorageController:
                 existing_state_dag_fwd_extrems,
                 new_state_dag_fwd_extrems,
                 # do not prune forward extremities in the state DAG
+                # else we lose eventual delivery
                 should_prune=False,
             )
 
@@ -946,12 +940,11 @@ class EventsPersistenceStorageController:
 
         Raises:
             SynapseError: if the new events include unknown prev_state_events
+            AssertionError: if there are no state DAG forward extremities remaining in the room
         """
         # filter out events which don't belong in the state dag.
         new_state_events_contexts = [
-            (e, ctx)
-            for e, ctx in event_contexts
-            if event_exists_in_state_dag(e) and isinstance(e, FrozenEventVMSC4242)
+            (e, ctx) for e, ctx in event_contexts if event_exists_in_state_dag(e)
         ]
         if len(new_state_events_contexts) == 0:
             # if there are no state events being persisted, then the fwd extremities of the state dag
@@ -973,19 +966,19 @@ class EventsPersistenceStorageController:
         ]
         # We want to check that we are not missing any prev_state_events.
         # To do this, we include rejected events in this check.
-        all_events = set(rejected_events + new_state_events)
+        all_new_state_events = set(rejected_events + new_state_events)
 
         # First, verify that we know all prev_state_events. If we fail this check then we don't have
         # a complete DAG and that is bad, so bail out.
 
         # Start with them all missing.
         missing_prev_state_events = {
-            e_id for event in all_events for e_id in event.prev_state_events
+            e_id for event in all_new_state_events for e_id in event.prev_state_events
         }
 
         # remove prev events which appear in all_events
         missing_prev_state_events.difference_update(
-            event.event_id for event in all_events
+            event.event_id for event in all_new_state_events
         )
         # the rest of these events should be present in the DB. Some of them may be forward extremities,
         # some may not be, that's ok.
@@ -1001,10 +994,9 @@ class EventsPersistenceStorageController:
                 room_id,
                 missing_prev_state_events,
             )
-            msg = (ev.event_id for ev in all_events)
             logger.error(
-                "_calculate_new_state_dag_extremities: was handling %s",
-                msg,
+                "_calculate_new_state_dag_extremities: was handling (max 10) %s",
+                [ev.event_id for ev in list(all_new_state_events)[:10]],
             )
             raise SynapseError(
                 code=500,
@@ -1041,6 +1033,8 @@ class EventsPersistenceStorageController:
             msc4242_state_dag_forward_extremities_counter.labels(
                 **{SERVER_NAME_LABEL: self.server_name}
             ).observe(len(result))
+
+        assert result, f"No state dag forward extremities left in room {room_id}!"
         return result
 
     async def _calculate_new_extremities(