Normally during seamless travel, the GameMode and GameState are only kept when traveling to the transition map, not when going from the transition map to the destination. If these actors are kept all the way through the seamless travel, it seems as though it causes issues with the client's characters, as they will be unable to control them after the travel is complete.
In the GameMode's GetSeamlessTravelActorList, move the calls to add the GameMode and GameState to the ActorList outside of the "if (bToTransition)" check.
Seamlessly travel from one level to another.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-140079 in the post.