diff --git a/synapse/storage/databases/main/events.py b/synapse/storage/databases/main/events.py
index 276188e21b..267b66272a 100644
--- a/synapse/storage/databases/main/events.py
+++ b/synapse/storage/databases/main/events.py
@@ -1572,7 +1572,7 @@ class PersistEventsStore:
                     DO UPDATE SET
                         {", ".join(f"{key} = EXCLUDED.{key}" for key in insert_keys)},
                         event_stream_ordering = CASE
-                            WHEN event_stream_ordering < EXCLUDED.event_stream_ordering
+                            WHEN event_stream_ordering IS NULL OR event_stream_ordering < EXCLUDED.event_stream_ordering
                                 THEN EXCLUDED.event_stream_ordering
                             ELSE event_stream_ordering
                         END
@@ -2005,39 +2005,39 @@ class PersistEventsStore:
                         event.internal_metadata.stream_ordering
                     )
 
-        # `_store_event_txn` is run before `_update_current_state_txn` which handles
-        # deleting the rows if we are no longer in the room so we don't need to worry
-        # about inserting something that will be orphaned.
-        #
-        # FIXME: We need to handle cases where we are persisting events out of order and
-        # the stream_ordering didn't increase.
-        self.db_pool.simple_upsert_many_txn(
-            txn,
-            table="sliding_sync_joined_rooms",
-            key_names=("room_id",),
-            key_values=[
-                (room_id,) for room_id in room_id_to_stream_ordering_map.keys()
-            ],
-            value_names=("event_stream_ordering",),
-            value_values=[
-                (room_id_to_stream_ordering_map[room_id],)
+        # This function (`_store_event_txn(...)`) is run before
+        # `_update_current_state_txn(...)` which handles deleting the rows if we are no
+        # longer in the room so we don't need to worry about inserting something that
+        # will be orphaned.
+        txn.execute_batch(
+            f"""
+            INSERT INTO sliding_sync_joined_rooms
+                (room_id, event_stream_ordering, bump_stamp)
+            VALUES (
+                ?, ?, ?
+            )
+            ON CONFLICT (room_id)
+            DO UPDATE SET
+                event_stream_ordering = CASE
+                    WHEN event_stream_ordering IS NULL OR event_stream_ordering < EXCLUDED.event_stream_ordering
+                        THEN EXCLUDED.event_stream_ordering
+                    ELSE event_stream_ordering
+                END,
+                bump_stamp = CASE
+                    WHEN bump_stamp IS NULL OR bump_stamp < EXCLUDED.bump_stamp
+                        THEN EXCLUDED.bump_stamp
+                    ELSE bump_stamp
+                END
+            """,
+            [
+                [
+                    room_id,
+                    room_id_to_stream_ordering_map[room_id],
+                    room_id_to_bump_stamp_map.get(room_id),
+                ]
                 for room_id in room_id_to_stream_ordering_map.keys()
             ],
         )
-        # This has to be separate from the upsert above because we won't have a
-        # `bump_stamp` for every event and we don't want to overwrite the existing value
-        # with `None`.
-        self.db_pool.simple_upsert_many_txn(
-            txn,
-            table="sliding_sync_joined_rooms",
-            key_names=("room_id",),
-            key_values=[(room_id,) for room_id in room_id_to_bump_stamp_map.keys()],
-            value_names=("bump_stamp",),
-            value_values=[
-                (room_id_to_bump_stamp_map[room_id],)
-                for room_id in room_id_to_bump_stamp_map.keys()
-            ],
-        )
 
     def _store_rejected_events_txn(
         self,