I’ve tried using experience from several different sources (killing mobs, breeding animals, etc). Also tried disenchanting the pickaxe and adding all new enchantments. I have a sword that I enchanted with Mending, and it appears to be working as intended. Only the pickaxe is giving me any problems.
Yes, I’m holding the pickaxe in my right hand. No other Mending items in inventory.
Resolved on my own. I was tapping the fish, instead of long-pressing on the fish.
I went back to an old world save and everything seems to be working correctly. This issue can be closed.