Pickaxe with Mending not repairing itself

I have a number of tools with the Mending enchantment. I have a zombie XP farm. Until the last update (I think, I don't think I used the XP farm during the previous update) when I collected XP with my tool in my hand the tool's durability would increase. This is going very, very slowly at the moment. My XP bar increases at a normal speed but the tool repairing has slowed to a crawl.