My opinion. At some point the inlay has become detached and it has been stuck back in the wrong orientation. I reckon if it was one 'turn' clockwise it would fit perfectly.
Those old inlays are probably thicker than you think. No chance there is another one underneath it.
Part of the reason it wont lie flat would be that the glue spot (which never seems to be placed centrally) is not now directly above the recess in the chip that it was when it was correctly oriented.