Had a similar issue. Was alive and well in mine about 10 blocks above bedrock. No MOBs or other threats in my tunnels. Had 75xp. Closed game and when reopened found I had fallen from aa great height and died on top of my fortress. Raced back to the mine to find all my items but they were not there. Headed back to my base on the surface and found all my items outside along with 7 xp. There was no real reason for having died, I was mining for diamonds and suddenly fell out of the sky????
I do not see how the issue was resolved. There is no explanation for why my character died by falling from a great height while in a mine 10 blocks above bedrock. All my gear was splattered on the surface outside my fortress