Hi, I have checked all my hardware keybinds again; nothing related to mixture is there. All damage is disabled.
I think I have managed to make this easily reproducible. First, a usb device (such as the throttle quadrant which controls the engine master switches) momentarily stops communicating with the computer for some reason. Maybe if the framerate gets too low and the game lags really bad? (Or if the USB is simply disconnected from the USB port. Then, when it gets reconnected, all the switches get reset to their "zero" position before returning to the position that is commanded by the throttle quadrant. This matches what other users have said in this thread and this thread.
What I have also found is that once the engines turn off, if the switches are quickly recycled on and off, and the ignition is set to IGN/START, the engines do recover, but once they get below a certain point, they will never turn back on again.
This should be easy to reproduce in the A330 or A321 (I haven't checked the A320neo, A310, or A400 but wouldn't be surprised if it's the same).
One more thing to note: this also happens with the spoilers axis and flaps axis (they get set to 0 momentarily when the usb device gets reconnected), but this does not happen with the throttle axis -- it remains in the same place when the disconnect happens. Maybe this is useful info.
I have attached a short video of this happening. You can hear the sound of the USB device reconnecting, then you can watch the engine master switches, spoilers lever, and flaps lever go to their 0 position, but the throttle levers do not move.
A321.mp4