Quote from magicjohansenI've experienced the same issue, however I was able to fix it. As I hate to disable my addons for troubleshooting I did it with all addons up and running - especially bartender4. If you open the options menu of bartender4 select the very top element on the left hand side called "Leisten" (I am Austrian ;)) so in English this should be bars or something like that. Then simply deselect the first option on the right side "Blizzard-Fahrzeug-UI verwenden". Should be in English something like use blizzard-vehicle-UI. Once you've done this you should be fine.
I hope this helps.
This solution does technically solve the issue, but I'm at a loss now for how I'd be able to view the vehicle frame if I disable it.