Kit Reveal: Jedi Master Kenobi

He lowers others, and so does the Rancor

From JMK's unique:

When another Light Side ally targets Jedi Master Kenobi with an ability, dispel all debuffs on him and he gains Mastery (stacking) equal to 10% of their current Mastery for the rest of the encounter

So if I have a toon with negative mastery through either SLKR siphon or Rancor, does he lose mastery?

My understanding is that if JKL is at -50 mastery, and does Jedi's Will and targets JMK, he will lose 5 mastery