well, if you upgrade to 100% activation rate then the effect activates with 100% chance ... you even said it got resisted, so it activated
resistance has nothing to do with activation rate
there are 2 checks ... 1st is activation rate, if the effect (debuff) activates then it goes to 2st check - acc x res check
Getting to 100% activation rate is far from uselsss...it makes you "skip" the first requirement step of activation, now you only have to get past the enemy resistance. Accuracy can help with that. Tough the only sure way to land a debuff is with a 100% activation rate with the additionnal mention "this debuff cannot be resisted".