Just user error. No bug.
-The next xp command does not support negitive numbers: /xp set player -10 points
Neither levels nor points work with negatives.
Chat error:
Integer must not be less than 0, found -10 at position 22: ...esAndSuch <==[HERE]
The 1.13 change log does say this is possible:
Negative numbers are supported, to subtract points instead.
Subtracting points can cause players to level down.
I attempted it with plenty of xp to subtract.-
You're running
/xp set– that would set their total exp to -10, which doesn't sound like what you want. Try/xp add player -10 pointsinstead. (Note that the section about negative numbers being allowed is under/xp add– not/xp set)