I had about 7 shulker boxes filled with netherrack waiting to be smelted into bricks laid out in my base. I died, respawned, and had to close the game and re-open it because entities were not loading (they were invisble, but still existing in the world) and items I dropped when I died were not visible (same, they weren't visible, but they did exist enough to pick them up.) Rebooting the game seemed to fix that, so I got my items back, but when I checked the shulker boxes they were all empty.

I suspect, though I am not a coder, that the game's way of handling entities is not able to properly store their data.