Mending doesn't always consume experience if the player wears items with mending that are already fully repaired

The bug

When using experience orbs to repair tools, weapons or armor that have been enchanted with the 'Mending' enchantment, some of the experience will not be used to repair the items if the player wears (armor slots, main hand, and off hand) any items that have Mending on them and already have full durability. Instead, the experience will be added to the experience bar.

Original description

While playing in my survival world with mending on all of my armor an tools when I'm repairing my shovel with exp it repairs very slowly when I have my armor on and I noticed only about 15-25% of the xp I pick up goes to the shovel. What I think is happening is that the xp tries to go to the armor (when it is being worn) but fails so results going into the exp bar. This may be intentional as I know if an xp is worth more than the durability left to repair it will go into the experience bar but I'm not sure if it was intended to be like that when the Item is at full durability. If it is somehow an unintentional feature then I personally like it because it means it's harder to repair items if you are just running around on the surface killing mobs.