Even in Seamless travel, most actors are destroyed because they live in the previous (now destroyed map) as you load the next one it (assuming you’re not ALSO using level streaming).
So that means things like PlayerState will be destroyed, then created when the next map loads, unless they are added to the list of actors to preserve into the next level. A few actor classes are added to this list by default, so you might not be able to use those for the technique I explain next.
So you can use BeginPlay on such a non-levelchange-surviving actor to check the possessed pawn class of the current player controller, and if it’s a spectator or not the one you want, you can spawn the one you want instead. Might be best to call a function in the Game Mode to make the selection and spawning happen at that point in the logic, but you can detect it from there.
But wait a minute, isn’t the Game Mode Actor itself destroyed when you leave one level and then enter the next?
So it might be as simple as saying there is no PostLogin event fired because in seamless travel the players don’t disconnect (logout), So you just have to make the PostLogin event call the same function that another place calls after seamless travel.