This bug is so annoying that I made an account just to report it. If you have a command block that changes the NBT data of a chest(for example, I have a chest the clones another chest 2 blocks above it with a kit in it full of items, so when you die and respawn, you can take the kit out of the chest and it will auto-refill the chest.) And this USED to work like a charm, but as of 1.21, if a command block changes the NBT data of a chest, it will kick you out of the GUI. So my system is now completely useless, because you can't even stay in the GUI for long enough to take one item out, much less, restock your inventory! Please fix, I need this!