Is the model oversized or extremely large? If so then the camera zoomed right up to the leg might reveal the cuts. I’m not sure why the lower LOD would be visible …
Quick tip in case the blend is oversized: make sure the transforms are applied in Blender using object->apply transform on the mesh, armature and all the nodes. Probably best to apply them individually in case blender chooses the first selected object.
Then you are probably ok with the default export settings - they usually reset for me after i reload the project. Also its a good idea to only export the selected objects, and for complex rigs check the ‘only deformation bones’ option.
Also ensure that the blend file has 1m per blender unit in the settings.