I know this question is fairly old but I noticed that there isn’t any answer here that doesn’t rely on a poll.
I think the best way to tackle (as you mention in a comment) is via a GameMode callback.
Unreal’s client-server model is focused around the concepts of worlds and their GameModes. A dedicated server will sit listening on a particular map with a GameMode loaded. When connections happen, the GameMode gets the chance to reject or accept connections with PreLogin, available when deriving AGameModeBase in C++ (this is probably also available in BP but I’m not sure of it).
After PreLogin is accepted we get PostLogin which will happen for you in the Lobby and I guess this is what is how you are detecting to initiate the Seamless Travel.
I think the callback that you are looking for is called HandleStartingNewPlayer. According to the GameMode docs, it is:
Called after PostLogin or after seamless travel, this can be overridden in Blueprint to change what happens to a new player. By default, it will create a pawn for the player.
So overriding this will give the server a notification after a seamless travel has occurred.
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