XP orb entities that collided with the player and were applied to mending, set their xp `Value` to 0, even if the stack `Count` indicates there is more than one orb present in this stack.
Simple solution would be to ignore the rest of xp if its not used in mending and decrease their 'Count' by one, or a more complex would be to 'unstack' that one extra orb with the remaining xp.
See attached analysis and video demo
https://youtu.be/BrVWQKV6uEA