Disregard my post about the corner of the little piece. This seems to be just a display angle visual thing and nothing wrong with the object.

It appears to be something with the surface normals of the larger object. They are inverted (hence the opposite interaction. MoI has a "flip" command, though if you seperate the surfaces, flip them, then Join them back again, the surface normals are reversed again. If you seperate the large object, flip the normals, then save it to a STEP file, then import the part and Join it together, it then Booleans properley and works as expected. This is why the mirror makes it work, because it is reversing the normals on the surfaces.

Perhaps something up in mirrored surfaces?

Was the original part a mirrored part Yennmonger?