Karlach leaving and not joining you right away is lame, imo.
She has a tadpole. Everyone and their grandmother is already after you, and her, because you are all infected. You're all going the same way. Why would she NOT join you?
I'm not sure, If that might get changed for release. Maybe they are still working in Karlach and that is the reason, she doesn't join at the moment. Finding the people responsible for her ending up in Avernus might then become her second act quest.
"We are all stories in the end. Just make it a good one."
