I managed to fix the issue on my side with a small modification to the engine source. In NetConnection.cpp, line 1814, comment out this line: ClientWorldPackageName = NAME_None;
This prevents the connection’s ClientWorldPackageName from incorrectly being cleared between the remote client’s player controller’s ServerNotifyLoadedWorld execution and the server’s PostSeamlessTravel call execution. Then, AGameMode::PostSeamlessTravel recognizes that the remote client was done loading the map before the server, and at then calls HandleSeamlessTravelPlayer for that client.
I’m not sure about side effects, but I think this is a good solution. Using logging I’ve seen that cases that would previously fail succeed with this change. Let me know if this solves your issue as well!