This has just happened to me, too, but much later in the game when I'm in the Underdark. It seems to be triggered by long resting, but there must be more to it than that as I've tried reloading a save from a few days and long resting and it doesn't happen then. I'll play around a bit and see if I can narrow it down, and also check back through the forum to see if this seems to be new for the new hotfix. (Oddly, searching BG3 forums for "following" doesn't throw up any results for me, despite this post clearly existing).
As you say, she doesn't follow or group sneak, though if she's the selected character then others will follow her. Saving and reloading doesn't help, and nor does long resting again, though I can reload saves from before the issue happened and she's following again. But when I reloaded the most recent save from before the issue occurred which for me was just after rescuing Baelen in the Underdark, it happened again as soon as I long-rested.
EDIT: In fact, here's a thread that looks as though it's related -
https://forums.larian.com/ubbthreads.php?ubb=showflat&Number=818924#Post818924. Not sure what I'm doing wrong with my forum search! Though from this thread it looks as though the previous issue was meant to be resolved with Hotfix 25 so perhaps this is different or has been reintroduced with hotfix 27.
