XP Sometimes Adds To Your Player Level Even If You Have A Mending Tool

My friends and I have experienced this but ever since 1.6 and after 3 major updates, I decided to report this because this is really annoying.

So, I have an xp farm that uses blazes. I use it for gaining xp levels and repairing tools. I go to kill some blazes, get the xp, and even though I have a tool (that needs repairing) with the mending enchant on it, the xp that I collect still goes into my player level

 

I have no idea if this is supposed to be happening, but It's very annoying because I have to take 3 times as long when I want to repair my tools