Furthermore, upon revisiting the code, it's currently not possible to add a negative amount of XP to characters from the XP Creator.
This suggests that if you are experiencing XP loss (not quite sure how at this point) it's probably not from the XP Creator, (based on recent testing).
In terms of your sharing XP theory:
For the XP Creator, XP is not shared, the amount of XP Gain is independently applied to every character you've unlocked.