Not a bug, mending randomly chooses between your main hand, offhand, and the armor slots. If the selected item is fully repaired it will add the XP to your XP bar rather than reselecting the item.
https://minecraft.gamepedia.com/Enchanting#Mending
Aaron Grosz
Not a bug, mending randomly chooses between your main hand, offhand, and the armor slots. If the selected item is fully repaired it will add the XP to your XP bar rather than reselecting the item.
https://minecraft.gamepedia.com/Enchanting#Mending