Realized that since I’m FF, I saw the original message but not Mark’s, and replied to the original message before seeing Mark’s. Later I saw Mark’s reply online and was momentarily flummoxed. (I didn’t receive it in email as a catch-up to the thread). This might point to some sort of hole with FF. I have to think about it.
This jolted my memory as I also FF+A on many of my groups, and I had noticed something similarly-weird some time ago but put it aside for the moment back then. So I just did some testing, and it turns out, when one is a Following-Only user in some group and have the "Auto-Follow-Replies" to ON, when they reply (by email or online) to some topic they were not following before, it will be added to their followed list, but no catch-up emails get sent, so it is a bug in the sense that the functionality isn't there. I just sent it as a #bug message to beta.