But when I turn on animation… its deformed
The fact your mesh appears fine means nothing.
For all you know the bones are 360deg around - which is probably precisely the issue based on the way it’s deformed.
Most likely, you passed a mannequin or ue4/5 thing into blender, with the wrong import options and ended up with a bad armature-skeleton.
Probably start from scratch in unreal. Delete everything. skeleton included. then check that stuff works right in a DCC, and if it does, then import a fresh FBX export with a fresh skeleton.
At that point, it won’t matter what the engine does, the new skeleton it creates on import should work no-matter what and will always be true to animations you export.
But there are bones without verts attached. Wouldn’t it be better to check weights first? 