I have a multiplayer world with several friends and one of them had two double chests break off into a single chest and a single invisible block that crashes when any player tries to open it. The newly split off single chests that are visible can still be accessed with no issue, with the items still inside. The invisible ones crash the world when you try to open it, but if you break them, all its contents drop on the floor like normal and you get the chest back. I've done some searching about this issue and found that it has been a bug before, but it appears to have come back.