I found two pages: https://forums.galaxy-of-heroes.starwars.ea.com/discussion/224131/galactic-legends-mastery#latest
https://forums.galaxy-of-heroes.starwars.ea.com/discussion/226093/developer-insights-supreme-leader-kylo-ren
To ensure that Kylo can Siphon Mastery even from units that don’t have Mastery, the amount of Mastery Kylo gains is based on his Mastery and Siphon stacks, and then the target loses the same amount that Kylo gained.
Note: Most characters have a base Mastery of 0 regardless of if they have their Relic unlocked. Mastery can go negative, and when it does, the stats that it affects for that particular character will be decreased rather than increased.