What are the odds of getting caldeus' skull?

[i am god]

I found it in the first bone pile I searched, unfortunately I fell down a pit so I stopped the game in task manager (thinking if I saved and quit it would leave me stuck there) and since the game doesn't autosave on acquiring key items I've lost it and haven't been able to find it again.
I've read that many people had a bug that simply wouldn't let the item spawn again so i'm wondering if, by chance, because of what happened, the skull would be marked as having been acquired?
Or does it simply have an astronomically low spawn rate and no easy way to find it? Even on easy, a full dungeon clear is like 30 minutes... what with all the ennemy spawns, having to fully explore every room to make sure you don't miss the specific bone piles, etc.
Do I really have to do the dungeon 20 times? Does it only spawn on impossible? Is the best option to just... forget about it and come back at the very end of the game if I absolutely want to complete the bestiary?

[Bron Kast]

I feel your pain. I have the Xbox version as my disclaimer. The drop-rate in my experiences (I have played through this game multiple times) have been pretty good. If not the first time, which is typical for me, then usually the 2nd time. If the game versions are the same then I doubt it would have to take more then 3 playthroughs tops. Good luck...