I went to the Nether to mine quartz to repair some tools, and my shovel with mending wouldn't mend when my only other mending tool was fully mended (my pickaxe). I went up 4 levels mining quartz with not a single orb repairing even 1 point of durability of my shovel in my offhand.
MC-238738
Holding a mending shovel in the offhand doesn't mend.
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The shovel within the screenshots you've provided doesn't have the mending enchantment on it, hence why it doesn't repair when you collect experience orbs. Simply apply this enchantment to your shovel in order for it to be repaired upon collecting experience. 🙂