When I put mending on my diamond armor and put on all my armor and collect xp to mend it up and I check my inventory to see if it's done mending it still shows that it has the same exact durability left. No matter how much xp I collect it still shows the same durability. If I take my armor off and put it in my inventory slot then it shows that it is fully mended and back up to full durability. Therefore it is mending up my armor but it is just not visually showing it until I take my armor off.