Bingo. It's definitely the phone. My USGS earthquake notification displays the same characteristic, but only on the phone.
Plus, I've tried twice to reply to this thread from the phone, and both times, it bounced. Even though I was replying from the email address I use for this account, the phone put the From address as my other email address, which is not a member of this group. It seems I can no longer (at this point, until they fix this) reply to groups.io messages from my iPhone.
I'm going to look around and see if this is a known bug.
Messages are the sole opinion of the author, especially the fishy ones.