When using citadel I noticed the bug. What happens in the plugin is a block is placed and then it takes a specified block out of your inventory as well which acts as a reinforcement. So lets say you need diamonds in your inventory when placing a stone block which is in your hotbar and placing a stone block. It removes the stone block and removes the diamond. Now lets say you are placing stone and need stone for it. You have two item stacks of Stone, one in the 1 hotbar postion and another one is the second. You are placing from the second. It currently takes from the first position and the stone being placed is gone as well as the additional stone.

The bug happens when you modify the itemstack that is currently being placed. So if you have stone in your first place hotbar and you need stone and then you place the block, the plugin removes the one stone but only one stone is removed from the player's hotbar.