So I've had this bug twice now, both with the initial release version and with the 1.01 Update.
Essentially what happens, either at cruise or this time while climbing up to altitude (though already fairly high) the plane just drops out of the skies, I get some kind of warning bell (not sure what, maybe just the stall warning), the autopilot (at least this time) did not turn off, when i turn it off and move the controls nothing happens. Both instances I was unable to control or stop this issue. After the first time it happened I thought maybe Icing was the problem, however this flight I had both engine and wing anti ice on.
I was able to capture most of the incident (paused shortly after it started). And as you can see the plane just falls out of the sky like a rock. I have uploaded the Video here (Sadly too large to upload here, 352mb) https://drive.proton.me/urls/5SY2C0Z9MW#1xzx2SUpUpuA
I have also attached a screenshot from Volanta showing the trajectory.
Let me know if I can be of assistance with any other information.