Diamond pick with mending, unbreaking III, fortune III and efficiency IV in main hand being used to break blocks will not repair and experience orbs go to experience level. If same pick is moved to off hand it is repaired.

I would expect an item with mending when held in the main hand to be mended.

With 4 items in inventory with mending enchantment, 2 armor pieces and 1 item in off hand and 1 in main hand all got mended except the main hand item being used to break coal, redstone and diamond blocks.

Steps to Reproduce:
1. enchant pick with mending
2. use pick in main had to break coal block
3. watch experience go to experience level instead of repairing pick