Throttle stop after front transition

I got a full vtol mission flight, all goes well except that, after transition to fw, my plane suffered a height drop about 3 meter(not fatal, but confusing!)
I checked my log, it shows that, the pusher throttle has a sudden drop after transition is done! My transition speed is set as 18m/s and I’m sure the motor is good enough for that.
here is my flight report

and I checked source code, and I found one function might be related to this problem:

For future reference, here’s the corresponding Github issue.