Ok, my last report on this matter.
As I suspected, mausoleum pieces DO NOT re-reswawn(pieces, which you already collected won't appear again).
Lately, I found 11th piece. Now it's gonna be really hard to find last one because of small chances to appear.
No bugs after all.
This ^.
Note that the chance of a given piece is not affected by the fact that the mausoleum pieces are UNIQUE. It just affects the chance of a respawn of a piece you already have (which has been reduced to zero). If the piece you need was going to spawn, it will still spawn.