I understand how frustrating this is. Me and are looking into it together. We will let you know what we find out.

If the graves were covering the skeleton, then that is not a bug. It has to be totally uncovered which is unfortunate if there are permanent items covering it.

and I tested this out, but I'm afraid that we did not find any issues with the achievement triggering when the Mortiking skeleton is fully uncovered. Due to that, it seems likely that there was some piece left covered if you didn't receive it.