TLDR I think it not changing your relationship with Kass currently is intended, and how it'll stay.
If you expose her on Varena's route she deliberately hams up her "YES, TIS, THE DEMON QUEEN" speech before dipping, and she makes a sly comment about seeing you later, so I get the impression she's not actually mad about it.
If you choose to out Kass on her own route (or rather tell her to fuck off because you're not keeping her secret any more), she seems much more annoyed, but all in all I doubt she's that upset. She has a line right before the decision where she's actively salivating over stealing your soul and turning you into a demon, so it'd be kinda fucked up for her to hold it against you.