I was playing a world today, and was experimenting with summoning large EXP orbs. The only mod I had was Fabric API 0.96.11+1.20.4 and MC 1.20.4

Expected:
Elytra/tools would mend

What Happened:
Elytra/tools went into the extreme negatives of the durability

Reproduce

1. Give yourself an elytra or tools
2. Enchant elytra/tool with Unbreaking 3 and Mending
3. Summon a large EXP orb a few times using /summon
4. Make sure the elytra/tool in in your hand and has < Max Durability
5. See that the elytra or tool has a extreme negative durability

Video:
click here, too big for GH

I did test this in vanilla, worked as expected.

Also works with other tools and other worlds

I also can kind of repro on 1.20.5. Instead, on 1.20.5, and when porting to 1.20.5, it just sets the durability to 0

If I'm correct, these commands should be able to reproduce this issue:

item replace entity @s weapon.mainhand with diamond_pickaxe{Damage:1,Enchantments:[{id:mending,lvl:1}]} # 1.20.4
item replace entity @s weapon.mainhand with diamond_pickaxe[damage=1,enchantments={levels:{mending:1}}] # 1.20.5

summon minecraft:experience_orb ~ ~ ~ {Value:100000}

In both Minecraft 1.20.4 and Minecraft snapshot 24w11a, I was able to get a near-broken pickaxe by running the above commands regardless of whether I was using vanilla or Fabric API.