Fellowing reproducible issue with 3.4.1 on the first testflight (Quadcopter, original Pixracer ) happened:
If one controls full nick forward, the copter shortly afterwards kicks back without reason.
Previous firmware was 3.4 beta rc 2 , flying without problems.
I can’t really help from the information you have provided but any Dev’s, or more knowledgable people than I, will need to see the logs from the test flights to analyse anything.