Mending

I have also run into this issue, but figured out a little more through testing.

If you are gaining exp to try and repair something with mending, the game has a chance to put the exp towards your levels instead of towards mending your item. I believe the reason is that if you are wearing other armor or have a tool in your main hand that has mending on it and is fully repaired, the game will try to mend that item instead, not be able to, and will therefore end up putting the levels towards your experience bar.

Obviously, this can make repairing tools 5 times slower if you're wearing armor, and it's really easy to reproduce.

Your description may not be clear due to the lack of details such as if you have multiple mending enchantments which will be explained bottom

Mending randomly choose between main hand, offhand, and armor slots. If selected item is fully repaired, XP goes to the player.

Hence, to be more efficient at repairing specific item, remove enchanted items from the slots that mending would randomly choose. For example, take off armor from the armor slot and or item from the offhand slot

Does that makes sense in your situation? Kolby

I have also experienced this problem in some worlds 

After a night of killing mobs the XP I gained could almost fully repair a almost broken item but now it can take me multiple minecraft nights to fully repair a almost broken item

@Eyeth is correct, if you have mending armor equipped and it is fully healed, then any XP that would go to healing that armor goes to your level. It seems that the XP allocation is chosen among all mending items, whether they are fully healed or not. Not sure if that is intended or a bug. (confirmed same behavior in 1.15.0.9 RTX Beta)

Thank you for your report!
However, this issue is Working as Intended.

The report you have submitted is working as intended: MCPE-36964.

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While this was WAI at the time of the report, the rules around parity issues changed in April this year and we are now tracking this under MCPE-80156.