diff --git a/changelog.d/4816.misc b/changelog.d/4816.misc
new file mode 100644
index 0000000000..43d94251f7
--- /dev/null
+++ b/changelog.d/4816.misc
@@ -0,0 +1 @@
+Add debug logger to try and track down #4422.
diff --git a/synapse/handlers/sync.py b/synapse/handlers/sync.py
index bd97241ab4..32101eb36a 100644
--- a/synapse/handlers/sync.py
+++ b/synapse/handlers/sync.py
@@ -39,6 +39,9 @@ from synapse.visibility import filter_events_for_client
 
 logger = logging.getLogger(__name__)
 
+# Debug logger for https://github.com/matrix-org/synapse/issues/4422
+issue4422_logger = logging.getLogger("synapse.handler.sync.4422_debug")
+
 
 # Counts the number of times we returned a non-empty sync. `type` is one of
 # "initial_sync", "full_state_sync" or "incremental_sync", `lazy_loaded` is
@@ -962,6 +965,15 @@ class SyncHandler(object):
 
         yield self._generate_sync_entry_for_groups(sync_result_builder)
 
+        # debug for https://github.com/matrix-org/synapse/issues/4422
+        for joined_room in sync_result_builder.joined:
+            room_id = joined_room.room_id
+            if room_id in newly_joined_rooms:
+                issue4422_logger.debug(
+                    "Sync result for newly joined room %s: %r",
+                    room_id, joined_room,
+                )
+
         defer.returnValue(SyncResult(
             presence=sync_result_builder.presence,
             account_data=sync_result_builder.account_data,
@@ -1425,6 +1437,17 @@ class SyncHandler(object):
                     old_mem_ev = yield self.store.get_event(
                         old_mem_ev_id, allow_none=True
                     )
+
+                # debug for #4422
+                if has_join:
+                    prev_membership = None
+                    if old_mem_ev:
+                        prev_membership = old_mem_ev.membership
+                    issue4422_logger.debug(
+                        "Previous membership for room %s with join: %s (event %s)",
+                        room_id, prev_membership, old_mem_ev_id,
+                    )
+
                 if not old_mem_ev or old_mem_ev.membership != Membership.JOIN:
                     newly_joined_rooms.append(room_id)
 
@@ -1519,30 +1542,39 @@ class SyncHandler(object):
         for room_id in sync_result_builder.joined_room_ids:
             room_entry = room_to_events.get(room_id, None)
 
+            newly_joined = room_id in newly_joined_rooms
             if room_entry:
                 events, start_key = room_entry
 
                 prev_batch_token = now_token.copy_and_replace("room_key", start_key)
 
-                room_entries.append(RoomSyncResultBuilder(
+                entry = RoomSyncResultBuilder(
                     room_id=room_id,
                     rtype="joined",
                     events=events,
-                    newly_joined=room_id in newly_joined_rooms,
+                    newly_joined=newly_joined,
                     full_state=False,
-                    since_token=None if room_id in newly_joined_rooms else since_token,
+                    since_token=None if newly_joined else since_token,
                     upto_token=prev_batch_token,
-                ))
+                )
             else:
-                room_entries.append(RoomSyncResultBuilder(
+                entry = RoomSyncResultBuilder(
                     room_id=room_id,
                     rtype="joined",
                     events=[],
-                    newly_joined=room_id in newly_joined_rooms,
+                    newly_joined=newly_joined,
                     full_state=False,
                     since_token=since_token,
                     upto_token=since_token,
-                ))
+                )
+
+            if newly_joined:
+                # debugging for https://github.com/matrix-org/synapse/issues/4422
+                issue4422_logger.debug(
+                    "RoomSyncResultBuilder events for newly joined room %s: %r",
+                    room_id, entry.events,
+                )
+            room_entries.append(entry)
 
         defer.returnValue((room_entries, invited, newly_joined_rooms, newly_left_rooms))
 
@@ -1663,6 +1695,13 @@ class SyncHandler(object):
             newly_joined_room=newly_joined,
         )
 
+        if newly_joined:
+            # debug for https://github.com/matrix-org/synapse/issues/4422
+            issue4422_logger.debug(
+                "Timeline events after filtering in newly-joined room %s: %r",
+                room_id, batch,
+            )
+
         # When we join the room (or the client requests full_state), we should
         # send down any existing tags. Usually the user won't have tags in a
         # newly joined room, unless either a) they've joined before or b) the