If you have armor with mending on it, it will still use xp to repair even if it is fully repaired. This results in slower repairing of tools that you hold, because most of the xp will be going to the fully repaired armor. I don't know whether or not this is a bug, but it sucks anyways because you have to take your armor off if you want to repair tools. This needing to take armor off got me killed in my hardcore world (rip).