2 files changed, 8 insertions, 2 deletions
diff --git a/changelog.d/15232.bugfix b/changelog.d/15232.bugfix
new file mode 100644
index 0000000000..d75a4f2d99
--- /dev/null
+++ b/changelog.d/15232.bugfix
@@ -0,0 +1 @@
+Faster joins: Fix a bug introduced in Synapse 1.66 where spurious "Failed to find memberships ..." errors would be logged.
diff --git a/synapse/handlers/sync.py b/synapse/handlers/sync.py
index fd6d946c37..9f5b83ed54 100644
--- a/synapse/handlers/sync.py
+++ b/synapse/handlers/sync.py
@@ -1226,6 +1226,10 @@ class SyncHandler:
                 continue
 
             event_with_membership_auth = events_with_membership_auth[member]
+            is_create = (
+                event_with_membership_auth.is_state()
+                and event_with_membership_auth.type == EventTypes.Create
+            )
             is_join = (
                 event_with_membership_auth.is_state()
                 and event_with_membership_auth.type == EventTypes.Member
@@ -1233,9 +1237,10 @@ class SyncHandler:
                 and event_with_membership_auth.content.get("membership")
                 == Membership.JOIN
             )
-            if not is_join:
+            if not is_create and not is_join:
                 # The event must include the desired membership as an auth event, unless
-                # it's the first join event for a given user.
+                # it's the `m.room.create` event for a room or the first join event for
+                # a given user.
                 missing_members.add(member)
             auth_event_ids.update(event_with_membership_auth.auth_event_ids())