I confirmed that if I swap Astarion out for a different party member, they will follow as expected. However, if I try to swap a second time to bring Astarion back, he still won't follow properly.
And the putative link to Group Hide could be correct, since I do use that feature fairly frequently. Alas.
This was my first real playthrough so I'm kind of sad about it, but I think I may just wait for Patch 8 to land for MacOS and hope I don't encounter the bug again.
