Great showcase of what the problem looks like, this got me looking at what I also believe to be the culprit. I’ll reply here whit my findings since the other thread has been resolved.
My experience after some testing is that the throttle axis (whatever binding you’re using, including “Throttle N Cut”) in this plane is defined incorrectly and entering the beta range when it should in fact be flight idle.
The tool tip, and likely the “Flight Idle” definition is misleading in the sense that the throttle lever can be in a relatively large range of motion and always indicate “Flight Idle”. To visualize this you can set the controller’s throttle axis to 0 and then use the mouse pointer and wheel to go up and down on the throttle lever.
Doing this, what I observed is that you can go up one click and down one click while always reading “Flight Idle”. Going up will set the behavior to the expectation of actual flight idle and controlled RPM values.
Both of the lower “Flight Idle” positions will result in entering the Beta range and over speeding the propeller. The following picture illustrates the throttle axis at value 0.
I understand this doesn’t directly answer the topic question but I believe it will help you track down the issue!
Extra info:
Looking at this comment, specifically at the propeller position it looks as if the dev team defined the throttle axis zero to be zero thrust and not the intuitive flight idle position