From my experience, if the furnace smelted many items and accumulated lot of experiences, sometimes the exp leaked to the player when collected even if the player wielded/equipped mending-enchanted tools/armors. I suspect the exp gathering animation/calculation was too fast so it didn't distribute the exp properly and caused slight leak.
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So is this being caused by the orbs being too large and your armor only having a small amount of damage? For me I just killed one skeleton but instead of repairing my axe which I was holding, I went from 27 to 28 levels. The axe was fully repaired except for the two damage points it took from attacking the skeleton, so it makes sense that that is the cause of this
What seems to happen is, that from the furnace you get xp orbs that give more points than a normal orb (simelar to the xp orbs when the ender dragon gets killed) and than that xp orb fully repairs one mending item, then realizes that it has still xp left to repair a mending item, but instead of repairing other mending items, the xp just goes into the xp bar simelar to MC-92889.
Reproduction Steps:
/give @s netherite_sword{Damage:10,Enchantments:[ {id:"minecraft:mending",lvl:1s}]} 1
/give @s minecraft:netherite_chestplate{Damage:10,Enchantments:[ {id:"minecraft:mending",lvl:1s}]} 1
Go into survival mode
Hold the sword in your hand
Put on the chestplate
/summon experience_orb ~ ~ ~ {Value:99
❌ Only one item got repaired and the xp bar got filled up.