This falls under the same issue as MC-87019, but while it was resolved for commands like /clear and /replaceitem, the mending enchantment still doesn't repair gear that is in a non-visible slot.

For clarification, I'm using non-visible slot to refer to a slot in the player's inventory that isn't currently visible due to having another inventory open. For example, offhand and armor slots aren't visible while looking in a chest (this is what made me notice the bug).