Interesting. What I can tell you is that I use the P3D command for Autopilot Master Off and it works fine. In fact it's worked fine throughout the Beta and all release and test versions.
If you haven't tried to bind your button to the P3D Command above, please give it a try. If you just have to had it assigned in FSUIPC (noting that we only recommend using P3D Controller assignments, particularly for trust levers) then you'll have to reach out to Pete in the FSUIPC forum.
Best wishes my friend!
This is a known issue with Chaseplane, and I've spoken to them about it. It's not an easy fix to do, so we'll have to live with this for the time being. I've been flying the this version of the Bus now for quite a long time, and found it easy to get used to.
Now, if you're not using Chaseplane then please let us know!
Thanks for writing in.