Umm, i really like the idea of experience boost but something is very VERY wrong with it.
When using it at my farm, i see the xp randomly dropping sometimes, thought it was weird.
Then when i finally got level 20 for my enchant i tried to apply it...and it voided my levels
all of them.
Back to level 0, but the anvil didnt put the enchant on the item.

On reloading the world when i picked up experience next i got my levels back somehow.
Whats causing the xp to randomly drop?

Not sure

I see the issue. It has to do with the delay of adding xp to the player via the experience boost status effect. I will need to re-write this.

This has been re-written and fixed in 1.21 and going forward.