When the player get some XP orbs while holding a tool or wearing an armor with mending enchantment, it constantly givng the xp to both the xp bar and to the tool/armor with mending enchantment at the same time making it inconsistent. You shouldn't get the xp to your xp bar before the tool/armor with mending enchantment is fully repaired.
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Can confirm, affects 1.17.2. It happened to me yesterday when I was mending my tools with xp.