On my Java world whenever I spawn in I have the same inventory.  If I place a block 10 blocks away, get out of the world.  When I get back in the block is back in my hand but 10 blocks away from me is the same block that I placed.  It doesn't save changes to my inventory or cords but it does save the world changes.  I think this might be my fault because I did 2 things.  It was in hardcore, when I died I opened to LAN world and changed my game mode back.  I then changed the world back to not LAN.  I also used NBTExplorer to go to the world files, go to Level.Dat, go to Hardcore.  Changed it from 1 to 2.  Then it was in survival not Hardcore.  I have a build I don't wanna lose but I'm not to far into the game.  So if I cant fix it I guess I will just have to make another.  I don't know what the "Affects version/s" means but my world is on version 1.16.5 and I'm on Windows.