Don't trust member list if source user isn't there

mautrix_telegram/portal.py

@@ -1146,12 +1146,19 @@ class Portal(DBPortal, BasePortal):
         # We can't trust the member list if any of the following cases is true:
         #  * There are close to 10 000 users, because Telegram might not be sending all members.
         #  * The member sync count is limited, because then we might ignore some members.
-        #  * It's a channel, because non-admins don't have access to the member list.
+        #  * It's a channel, because non-admins don't have access to the member list
+        #    and even admins can only see 200 members.
+        #  * The source user is not in the chat, because that likely means it's a group
+        #    with the member list hidden (so only admins are visible).
         trust_member_list = (
-            len(allowed_tgids) < 9900
-            if self.max_initial_member_sync < 0
-            else len(allowed_tgids) < self.max_initial_member_sync - 10
-        ) and (self.megagroup or self.peer_type != "channel")
+            (
+                len(allowed_tgids) < 9900
+                if self.max_initial_member_sync < 0
+                else len(allowed_tgids) < self.max_initial_member_sync - 10
+            )
+            and (self.megagroup or self.peer_type != "channel")
+            and source.tgid in allowed_tgids
+        )
         if not trust_member_list:
             return None