I say hex is bugged

but I guess then if you use grease. then she will have a hard time to stand up and then your sacred flame will hit

ah no,

its a saving throw too , so tell me, ? what is the point of this debuff on Ability check ?
I think it's more likely that the wiki has an incorrect description. After all, Larian isn't the one who is writing the wiki!
Larian has already changed prone to allow 100% chance to hit with melee attacks. It makes sense that, similarly, dex ST spells would have 100% to hit.
Prone, 5e rules-as-written, doesn't give penalties to Dex STs. I think this is an example of a good decision Larian has made that allows Sacred Flame to be more powerful. Once (if) Larian implements the ability to Shove people prone, instead of just pushing them away, Sacred Flame should become much more useful