This happens only when a user follows a new (to them) topic, which tells the Architect to email right away the OP post and any existing replies to the member so they can catch-up if needed, at least for the Individual Email users, with either All-Messages or Following-Only & First-Messages-Also & Follow-Replies. (I only tested those two, so not sure if it will do it for digest, etc)

The email subject is not populated with the group's [Subject Tag] on those emails, but it should, shouldn't it?

Cheers,
Christos