I have a mending fishing rod in my survival world and after using it for a while I noticed the durability declined instead of increasing like on the minecraft wiki. I have no other mending tools in my inventory and only some of the experience I gain while fishing goes to it (some goes to my xp bar), it seems as if some of the xp does not register to the rod or my bar.
Edit I did some testing and it seems that occasionally fishing rods do not even give experience and it seems to grow more frequent as I level up, this seems to be the cause of the mending rod decreasing in durability and my bar seeming to ignore the ex I thought I got.
edit I belive that I have foundone he underlying problems. First off mending seems to ignore the xp gained while fishing, second I found a solution to the xp loss in my bar, if I fish and get no xp, switching to another hotbar slot then back to the rod seems to add the missing xp to my bar.

I am using a Galaxy Tab A 6