We have this problem, after importing the character from Blender, one leg is rotated by 180 degrees, which compromises everything. Setting Preserve Local Transform during import does not help.
this is how it looks in blender
this is in UE Mesh window, make the note that the mesh is ok, but the bone is upside down
and this is how the animation is inside the engine. The bone is placed well, but the leg is rotated