Resistance is the percent chance a debuff isn't applied. For example, if a character has 50% resistance, half of all debuffs won't be applied to them. It doesn't matter what type of debuff it is. Permanent, conditional, timed; it doesn't matter.
For dark might, he applies 2 debuffs (i think 3 stacks of a single debuff counts as a single debuff, but a dev would need to confirm). If one of them isn't applied, it's most likely because of resistance.
I'm not sure if there's a bug here. If you have video of this occurring without the opponent resisting, please share it. In 6.3 we'll be adding Replays to the game, so if it happens again in a fight, you can share the replay code as well.