okay this has been bugging me for like two weeks now. replaced the front right motor after a pretty minor crash, honestly thought it was going to be an easy fix. but ever since, the prop on that motor keeps working itself loose mid flight. i've lost three props already, one of them flew off during a slow cruising pass which was embarassing and also kind of scary. the other three motors are completely fine, i never have to touch them. the new motor shaft looks normal to me, im using the same prop nuts i always use, same torque roughly as the others. is there something about a new motor shaft that could cause this? like does it need to be broken in or something? or could the threads be slightly different

Check if the shaft diameter is actually the same. Sounds obvious but ive seen "5mm" shafts that are slightly undersized from certain suppliers and the prop adapter just doesnt grip right even when torqued down. Also make sure you're using the correct rotation direction motor for that position ? if someone shipped you the wrong one and you just reversed it in software the prop nut is actually threading the wrong way for the direction it naturally wants to unscrew under load. That would 100% explain this.

the rotation direction thing the other guy mentioned is probably it honestly. self tightening prop nuts only work if the rotation direction matches how they're designed, if its reversed in betaflight instead of being the physically correct motor the nut is basically just unscrewing itself every flight. happened to my buddy and he went through like six props before someone figured it out at the field