I have noticed a similar issue that others have reported with the mending enchantment on the pickaxe. I wanted to add some further information.. and can provide video.. however it will take several hours of data. – It seems that the pickaxe will repair in your off-hand slot, as long as no other mending items are on you. (I.E. Naked character) However, if the pickaxe is in the hotbar, or otherwise on your person.. every other mending item is randomly selected Except the pickaxe. I do not know game code well enough to figure out if this is exclusive to the diamond pick or the enchantments that I have on. Also the only modifications to my game session are a custom made resource pack (textures) and no Mods are installed in this version. (No optifine or other add-on). This might be intended, but I personally have spent 2-4 hours in front of a spawner XP farm and gotten zero repair on this item, while all my other items repaired. Only when I took everything out of my inventory, and put it in my off-hand it repaired every time i collected XP orbs. (But lets be honest, killing mobs this way is a pain). Thanks for your help.