Mizaar

Hi guys. Does anyone know how to fix this issue? I try to bound custom player model to a rig and it happens (screenshot). I work in blender and know about its problems with bones, but I get same result in 3d max aswell. (but im absolute 0 at max). I even tried make Y-up on export. I tried ingame armature (it's broken ofc). I tried rig from someone's blender project (same result). I remade armature from scratch - same result. Every time I replace stock model with other one or join a head with bool tool - it brokes. I miss something but I have no idea what to google alredy. PS Ive been struggling for a week already pls help PPS I use furry mod cuz it allows me to use custom player model instead of changing every humanoid in a game