Pretty sure that used to happen. Ironically I'm able to make use of the one in the Blizzard bar that's still showing up in that scenario so maybe keep that issue around until this one gets fixed. :)

Is this still an issue?

Depends on how it's supposed to work, the dedicated Vehicle Exit button exists and it's been a while (I think) since I noticed that I didn't have that button active while in vehicle mode so it may be okay now even if the button isn't added to the vehicle bar the way it used to be.

Yeah I've tried to effectively make the "Exit Button" be the de-facto way to exit vehicles. There are still situations though where the possess/vehicle/override buttons get their own unique exit button, but that is up to how Blizz codes the quest. I think only place this doesn't work well is during mind controls, but it's easy enough to 'esc' out of the MC, and it works about how I think the default UI works.

I'm going to close this for now, feel free to reopen if you still see issues

Yeah, that's fine. I did get a vehicle bar recently where the 10th button was a "special" (return to base or somesuch), I think that's where the exit button used to go but I think the current behaviour is fine.