Mending Tool/Armour Selection

I have a very good blae farm that provides me with loads of XP. All my tools and armor have Mending on them. I read what mending does, and it says that it selects one item randomly. But, if I hold my sword, only my armor and the sword in holding receives the XP. And the pickaxes that are in my hotbar don't receive XP unless I quickly switch and hold them, which is really annoying. Is this a bug?