I think perhaps the mail providers have become more aggressive classifying emails as spam. Today I started seeing replies to an email chain where I did not receive the first message, even though I was clearly in the to: list. When I logged into the online email (spectrum) I found three emails from the same person in my spam folder. We had emailed back and forth many times in the past so I have no idea what triggered it. And the replies from the other to: folks (that had simply replied all) were not classified as spam, even though the topic was now Re: <original topic> .
So then I decided to review my yahoo email spam. Sure enough there were several that should not have been spam, including a "Welcome" message from another group here at groups.io. But funnier still, although Yahoo marked the welcome message as spam, I received a half dozen other messages from the same group without problems.
It sounds complicated, but EVERYONE should be checking their email account for email incorrectly marked as spam. It doesn't just affect groups.io. This is far more pervasive.
Tom Link