I contacted support and was asked to put the bug I found here. I am updated to the newest version 1.2.13. So when I'm on the realm and any item I hold has mending on it the item will absorb the exp but it will not gain durability. Instead all it does is bounce up and down at high speeds with it's durability bar fluctuating, someone on my realm even claims to have had their fishing pole duplicate during this bug into their inventory but I cannot confirm that. I can confirm however that this happens with fishing poles and mining picks and ONLY on realms. When I take the same world and download a copy to my PC and run the same world and go to my mob exp farm the pickax does not bounce after absorbing the exp and the durability is replenished just fine. It is HIGHLY repeatable and has become a serious problem with trying to repair items on my realm. Sorry I cannot confirm the duplication or any other items as I do not have any other items with mending besides armor (which has no issue with mending).