I build a quad-x Biplane tailsitter in ArduPlane V4.1.2 firmware. One of the four motors pwm dropped periodically until it stagnates at 1150 in qstabilize mode (with no props) and this happened alternately on one of the motor when retrying.
Here are the log and params
https://drive.google.com/drive/folders/1Q73Hxfim34VFyMO1wrAoPTOlP4rEic8D?usp=sharing