Steps to reproduce:
1. Equip armor without full durability and with mending
2. Put mending weapon without full durability in main hand
3. Try to collect xp orbs from mobs or other methods and observe
Observed Results: Not all xp orbs go to mending items with unful durability and instead some go to the xp bar.
Expected Results: Xp orbs are to go to equipped items that have the mending enchantment before allowing the orbs to affect the xp bar

Yea I'm having the same problem. On my Xbox One when I have things equipped such as my armor and weapon in my main hand, all having mending, it seems to only at chance repair one of the things. Specifically, I'll have full mending diamond armor and a mending diamond sword, I'll receive the xp orbs but instead of them all going to unful durability items such as my equipped armor or my main hands weapon, it will randomly at most times go to my experience bar leaving my armor and weapons with unful durability. Taking off the armor seems to work with making sure the experience goes to the equipped item but this is extremely inconvenient and not how mending is supposed to work. It should repair randomly rather a piece of mending armor or mending item equipped with unful durability before allowing the experience to fill up my bar.