Hi!
Currently struggling with this bug so thank you so much for providing the report and a workaround.
Unfortunately we don’t use a transition map and I have no knowledge of how this should look, do you know what it should look like for someone not using a transition map?
This bug was indeed fixed in 5.1, which was released!
“LastCompletedSeamlessTravelCount == SeamlessTravelCount” check was removed from APlayerController::HasClientLoadedCurrentWorld to prevent issues when client finishes travel before the server.