Is it possible that you have selected reverse when the OD is still engaged. I know the inhibiter should stop that but dirt can also jam the valve in the OD and cause it to stay on. It might leak down slowly for a while and be OK if you don't go into reverse for a while after switching it off but at some point will catch you out. I don't know what actually breaks in the OD or box.
Coincidently Rob Henchoz mentions this in his latest video on YouTube, referencing a competition PV that has OD and the owner has messed up the box.
https://www.youtube.com/watch?v=_SAOUVwOTyo