So I am killing the Endermen at my Enderman Farm. I repair my tools using the mending enchantment. But what I noticed is that the damaged tool repairs slower if you have an armor piece/tool (offhand) equipped.

You notice the tool bouncing down, once you collect a experience orb?
That's when the NBT data changes/durability value increases.
You can also look at the durability tooltip after pressing F3+H.

1. Put armor on with mending on all pieces.
2. Hold a tool with mending in one hand and a damaged tool in the other.
3. Pickup experience and see that the damaged tool does not absorb the experience every time.

4. Clear your inventory. Only having that damaged tool in your hand.
5. Pickup experience and see that every orb gets absorbed into the damaged tool.

1-3 show that some experience not absorbed by the damaged tool gets stored in the xp-bar.
4-5 show that all experience gets absorbed into the tool.

What is the bug here?
Damaged items not absorbing every orb with other mending pieces equipped? OR
Damaged items will always absorb xp when no other items with mending equipped?