We got caught off guard with this too since the official documentations tates that game modes are persistent
Persisting Actors across Seamless Travel
When using seamless travel, it’s possible to carry over (persist) actors from the current level to the new one. This is useful for certain actors, like inventory items, players, etc.
By default, these actors will persist automatically:
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The GameMode actor (server only)
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Any actors further added via AGameModeBase::GetSeamlessTravelActorList
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All Controllers that have a valid PlayerState (server only)
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All PlayerControllers (server only)
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All local PlayerControllers (server and client)
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Any actors further added via APlayerController::GetSeamlessTravelActorList called on local PlayerControllers