Chests and anvils can be manipulated as they dont have a fixed collision hitbox. Standing against an anvil and looking a different anvil thats rotated will cause the hitbox of the anvil ur against to change to the one ur looking at clipping your hitbox inside the anvil. Same thing happens with chests. https://www.mcpk.wiki/wiki/Anvil/Chest_Manipulation

1. The fixes have been implemented in ViaFabricPlus instead as ViaFabric can't fix these ones.
2. Also not possible to do as it's a collision box change and only works in singleplayer.