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.-