I have used groups.io for a couple of years - a refugee from Yahoo's groups. I read the posts to my groups via my browser, not via email. I use Windows 10, Firefox latest edition of browser.
I read Messages/Messages style, versus Message/Topics. In the past, whenever I have looked at a post, whether by selecting it from the whole list of messages or by moving to it via the next message/previous message buttons visible when reading any single message, the title of the post turns gray, indicating I have read the post. Unread posts show titles as blue. This gives me a visual indication of whether or not I have read all the messages in my group when I select the group for display.
My habit has been to scroll down to the oldest unread message, then see them all, one by one, by clicking the "next message" button, until I reach the most recent message. In the past all the messages will show a gray (i.e., "read") title when I have completed the list.
In the last 48 hours or so only posts that I have selected to read from the overall list of messages turn gray; any that I read using the next message/previous message buttons within a message retain their blue (i.e,. unread) title.
Anyone else notice this behavior? Is it a feature or a bug?