If the tooltip says 220 accuracy is what you need and the ability says 100% chance to debuff or cc, then it should be 100% and not 97 or 99.7%.
I experienced a lot of inconsistency with debuffs, sometimes i had overshot the accuracy by 100 and more and the defuffs were still resisted.
Even when perfectly achieving the suggested accuracy this does not change and it always feels really bad to lose because the game is inconsistent when it comes to accuracy.
I took time off because i got so frustrated with this problem when i worked on the season 1 dragon bosses where it took me around 2h to complete them with gear were i knew that i win if my debuffs land.
I am sure that a function is in place like this:
f(x)=x/(x+a) or f(x)=1−(1/(x+1))
but please use one which can scale up to 100% and if the function would return over 100 it would be returned as 100, same with 0