I've noticed that when a grindstone is used to unenchant items, while it is being placed against a wall facing west, the XP orbs are getting stuck behind it. This happens regardless of the orientation of the grindstone (placing from the front or sides, or on the wall), as the XP always seems to be emitting in the negative X / West direction. For instance, if the grindstone is placed against a northern wall, the orbs will come out of the left side and not get stuck.