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? 
Nha. Seems like inverted bones based on the twist of the right arm/left leg. Call it an educated guess?
But sure. Weights working are also a must - should be checked in the dcc.
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For the future I hope it helps somebody else to not waste time debugging and fighting against the Engine. The fragile moment in PSK path: When we swap the skeleton pointer AFTER the mesh is built, Section.BoneMap was already baked with indices into the throwaway skeleton’s bone order. Our reconcile + Build() is supposed to rebuild it against master, but UE’s rebuild logic relies on the mesh’s imported data being internally consistent — and the Gildor (an app)factory’s throwaway skeleton + per-vertex BoneIndex pair IS internally consistent, so Build() may not re-resolve BoneMap via name lookup; it just re-emits what’s there.
FBX skips all of this because the skin clusters are named-bone-references from the start. UE’s importer maps them to the target skeleton by name at the very first step.
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Thank you for the answer! Appreciate the interest of helping!