Re: A new, but still tiresome, way to reply to messages - Trying to "reply to" messages, without success!
After a bit of digging I think I may have found the cause, and solution, to your problem.
If you receive the daily digest in plain format, hitting the link after "Reply to this message" for a given message throws you into your web browser with the correct groups.io thread open for response.
If you receive the daily digest in full featured format, hitting the "Reply to Group" link at the end of a given message causes your e-mail client to be triggered, a compose window opened, and the subject prepopulated with "Re: <message subject that you're replying to>" and the To: field prepopulated with the correct group e-mail address. You can tell if you have the plain format daily digest because these open with the words, "This is a digest for," while the full featured format opens differently and includes a Graphic at the top and reads differently at the beginning.
By the way, this seems completely counterintuitive to me. Were I a plain text digest recipient I'd probably have chosen that because I want to use e-mail as my primary interaction method while if I were I full featured digest recipient I'd be far more likely to be using the web interface (or at least as likely as e-mail).
You can very easily test this out by changing your digest subscription. Try flip-flopping from plain text to full featured and see if you get what you've been looking for. The e-mail instructions to do this are:
Email Delivery Options
To receive plain digests instead of individual messages send an email to:
To receive full featured digests instead of individual messages send an email to:
To receive individual messages instead of digests, send an email to:
To stop receiving messages via email (you may still read messages on the Web), send an email to: