Well, hopefully as advertised hotfix 10 did fix the follow issue with Shadowheart, but it seems to have made Halsin worse

He now no longer starts following my party again on reload. On the plus side, this has made it possible to send a save game showing the problem to Larian so I have now submitted a bug report. On the minus, this takes having him in the party from inconvenient to unplayable. Ah well, there was a hotfix for Shadowheart, so hopefully if the Halsin bug is now replicable there'll be one for him, too!
EDIT: Hmm, actually Halsin wouldn't follow my party in Owlbear shape, but after he turned back into an elf he started following again. I'll keep him in the party for a bit longer to try to work out what's going on!