Okay, I've played around a bit and found out more, though I still don't understand exactly what is triggering this. In every save I have back to (and including) one from 14:10 BST on 27 July, if I long rest then Lae'zel stops following me. The save I have before that on is half an hour earlier, at 13:41 BST on 27 July. That is in the Zhentarim hideout after wiping out all the Zhentarim. I can't recall exactly what I did last time between those two saves, except enter the Underdark via the hidden lift in the hideout, kill two Minotaurs and start walking towards the Myconid colony. I've tried doing that, and got as far as the colony and spoken to the king, long resting frequently and Lae'zel is still following.
I don't really fancy doing everything I did between that save and today again so may wait and keep my fingers crossed for an swift hotfix for this one!
