fix(ui): SlidingSync::subscribe_to_room marks room as having missing members.

Fix https://github.com/matrix-org/matrix-rust-sdk/issues/2004.

crates/matrix-sdk-base/src/rooms/normal.rs

@@ -146,6 +146,11 @@ impl Room {
         self.inner.read().unwrap().members_synced
     }
 
+    /// Mark this Room is still missing member information.
+    pub fn mark_members_missing(&self) {
+        self.inner.write().unwrap().mark_members_missing()
+    }
+
     /// Check if the room states have been synced
     ///
     /// States might be missing if we have only seen the room_id of this Room
@@ -680,7 +685,7 @@ impl RoomInfo {
         self.members_synced = true;
     }
 
-    /// Mark this Room still missing member information.
+    /// Mark this Room is still missing member information.
     pub fn mark_members_missing(&mut self) {
         self.members_synced = false;
     }

crates/matrix-sdk/src/sliding_sync/mod.rs

@@ -134,7 +134,14 @@ impl SlidingSync {
     }
 
     /// Subscribe to a given room.
+    ///
+    /// If the associated [`Room`] exists, it will be marked as members are
+    /// missing, so that it ensures to re-fetch all members.
     pub fn subscribe_to_room(&self, room_id: OwnedRoomId, settings: Option<v4::RoomSubscription>) {
+        if let Some(room) = self.inner.client.get_room(&room_id) {
+            room.mark_members_missing();
+        }
+
         self.inner
             .sticky
             .write()