only enable translation logic on pseudo ID rooms

- see added TODO for more info

clientapi/routing/state.go

@@ -217,6 +217,18 @@ func OnIncomingStateTypeRequest(
 	var worldReadable bool
 	var wantLatestState bool
 
+	roomVer, err := rsAPI.QueryRoomVersionForRoom(ctx, roomID)
+	if err != nil {
+		return util.JSONResponse{
+			Code: http.StatusForbidden,
+			JSON: spec.Forbidden(fmt.Sprintf("Unknown room %q or user %q has never joined this room", roomID, device.UserID)),
+		}
+	}
+
+	// Only enable translation logic on pseudo ID rooms
+	// TODO: remove the if check, synctypes.FromClientStateKey handles non-pseudo ID rooms,
+	//       but has some logic to be worked out (see comment in synctypes.FromClientStateKey for details)
+	if roomVer == gomatrixserverlib.RoomVersionPseudoIDs {
 		parsedRoomID, err := spec.NewRoomID(roomID)
 		if err != nil {
 			return util.JSONResponse{
@@ -245,6 +257,7 @@ func OnIncomingStateTypeRequest(
 			}
 		}
 		stateKey = *newStateKey
+	}
 
 	// Always fetch visibility so that we can work out whether to show
 	// the latest events or the last event from when the user was joined.