This bug may be a feature, but it doesn't seem likely. Mending works differently depending on if you have mending gear equipped. It randomly chooses one equipped item to repair. If you wear mending armor, than it may choose a fully repaired piece, and then the exp is wasted on the xp meter instead of your tools or armor. I believe this should be fixed so it will always choose the most damaged of your equipped gear until everything is fully repaired. Unfortunately I cannot give any screenshots because I am playing on a ps4. Thank you.