I was attempting to fight the Wither Boss in the nether when he escaped containment and killed me. By the time I got back to the area, my grave was exploded and my items (plus the grave-block item) were strewn across the ground.

As it didn't pop while I was still in death screen, but was by the time I got back, my assumption here is that it was caught in the blast radius of one of the blue skulls.

Interesting, since the block is marked indestructible. I'll try to take a look at it when I get home after Christmas.

See message on other issue.