Hello GameProject, Welcome to the Forums.
How confident are you that the First Person Device has always remained in first person when entering a chair? I believe it has always kicked you back to third-person. (In the same way that it does when entering a vehicle or turret)
I can think of some workarounds involving invoking other cameras based on the Chairs ‘On Player Seated/Exited’ events if you want to go that route. (Not sure what you mean when you say your map is inaccessible though)