This is an old problem but seems to have gotten worse . When logging in or relogging while standing on a floor wood, stone ect any type of floor even single stone cube on the ground Players end up trapped under floor and usually die. Sometimes if your quick enough you can destroy the block your trapped under and escape but if not quick enough you die and have to respawn in bed and go to the area and destroy the floor to reclaim your bag. My personal solution is to only log out on grass or dirt outside of build but this is not always possible.