2. Got this replyI think this results from a change Mark made some years ago, to use the group's posting address as its "name" or identification in the various notices.
I'm not really a fan of this change, in part for the reason you've encountered, but also just because it looks ugly to me.
Part of what motivated this change was the challenge of citing the "name" of a subgroup. Because they have two: the primary group (subdomain) name and the (sub)group name. Using the posting address combines them in a way that is superficially similar to a non-subgroup address:
Myself, I would have dropped the groups.io part when used as a name (identifier), eliminating the confusion with an email address:
subdomain | name
Both of those are the forms used in the Your Groups list.