Yeah, if the taxonomy has “First Post Only” enabled (meaning only the first post in each topic in that taxonomy will be federated, then content is intentionally attributed to the taxonomy itself instead of the user. This allows for the use case where you have an “Announcements” category or similar you will federate content as the Group actor (which is normally what people want in that scenario). In the Discourse plugin, the “standard” use case you’re referring to is the publication type “Full Topic” (which is what all the other actors on this site are set up as).
In this case, the reason the tag was initially set up as “First Post Only” was because there is an admin topic control that allows you to force-publish a topic, even if it’s already posted (as was the case here), however to reduce complexity that feature was initially restricted to “First Post Only” taxonomies. So I initially set up the tag like that, (followed it) and published the topic to make that work. I’ve since switched the tag to “Full Topic” which is what most people will expect in this scenario. If someone replies to that topic, the reply will be processed.
Actually, since doing that, I’ve already made a note to remove that restriction on the force publish feature (i.e. to allow for it for Full Topics), which I’ll be pushing soon. So in short, that’s intentional, and the need for it in this case was a temporary practical one.