Throttle fluctuates to 0% and back on mode change

I have a Tilt rotor quadplane here, Ardupilot 4.3.1

Aircraft transitions and flies correctly in FBWA. However, upon changing to Auto, (or presumably any other mode with auto throttle) in some fraction of a second the throttle output plummets to 0% before returning to a controlled value, which is very difficult for the motors and ESCs. This is despite the default 100% setting of the throttle slew rate.

Is this a known behavior? Is there any way to prevent this?

Thank You

Please send the log in here.

Here the log

After the blip it was dead stick because the ESCs desynced