On the 8th Floor of the dungeon in the betweenlands there are moving walls that destroy anything in there way. They will also kill you instantly no matter your health if you get pinned by them. Your grave will spawn and then in turn also get destroyed by the wall and you'll either lose all your stuff or you will not be able to get down to them in time to get your stuff before they despawn due to the grave being broken. Don't know if this can be fixed though. Maybe a way to make graves unable to be destroyed.

This commit will allow this to be fixed Angry-Pixel/The-Betweenlands@1037fa5 when it's released

A new version with a blacklist for the moving walls breaking is now available https://www.curseforge.com/minecraft/mc-mods/angry-pixel-the-betweenlands-mod/files/2835615