I placed a minehole some time ago like always, saving the spot with towercap boards. Today I placed the rock crystal and steel inside since this is for -7. There were quite some number of cave lice and slime around, so I waited for the grid to unload before returning with more building materials -- 11-12 minutes on w16.1. Upon returning, the minehole building sign was missing the steel and rock crystal I had just placed into it. It couldn't have been a decay tick because the sign wasn't destroyed and the towercap boards were still recorded within it. Of course, the steel and rock crystal are not in the locations I retrieved them from before building with them.
I can only theorize of course, but is there a limited time window allowed for persistence and unloading the mobs took longer than this window? There was like a dozen cave lice and slimes each.
Everything since has been saving appropriately of course
https://ibb.co/BVCWjzmj