diff --git a/changelog.d/20438.bugfix b/changelog.d/20438.bugfix
new file mode 100644
index 0000000000..0e8aed3c04
--- /dev/null
+++ b/changelog.d/20438.bugfix
@@ -0,0 +1 @@
+Fix a bug where a client using `state_after` ([MSC4222](https://github.com/matrix-org/matrix-spec-proposals/pull/4222)) would keep showing a room as invited or knocked after the user rejected the invite or rescinded the knock.
diff --git a/synapse/handlers/sync.py b/synapse/handlers/sync.py
index d635465e5c..3ada731e93 100644
--- a/synapse/handlers/sync.py
+++ b/synapse/handlers/sync.py
@@ -3570,6 +3570,25 @@ class SyncHandler:
                 # An out of band room won't have any state changes.
                 state = {}
 
+                # MSC4222 clients do not apply timeline events to room state, so
+                # the out-of-band membership has to appear in `state_after` or they
+                # never learn that the user rejected the invite or rescinded the
+                # knock. Sliding sync does the same in `get_room_sync_data`.
+                #
+                # `events` is unfiltered, so a client filter that hides the
+                # membership event still gets it in `state_after`. A filter that
+                # empties the timeline batch gets no room at all, as the shortcut
+                # above returns first (see the FIXME in
+                # `tests/handlers/test_sync.py`).
+                if sync_config.use_state_after:
+                    for oob_event in events or ():
+                        if (
+                            oob_event.type == EventTypes.Member
+                            and oob_event.state_key == sync_config.user.to_string()
+                            and oob_event.internal_metadata.is_out_of_band_membership()
+                        ):
+                            state[(oob_event.type, oob_event.state_key)] = oob_event
+
             summary: JsonDict | None = {}
 
             # we include a summary in room responses when we're lazy loading