I don't know if this is a sign of a problem. All my instincts tell me it is user-error, but just in case it isn't, I'm reporting it here.
I have a group member with two apparent but related problems:
(1) He keeps finding his subscription changed to one of the Daily Digest or Summary settings. He wants individual messages only.
(2) He keeps finding himself unsubscribed from the group. This happened several times today alone, but before too.
Does he forward group messages to other people? If so, they could be clicking on the Unsubscribe link in the message footers.
There is one other oddity. He says his email gateway (where he works?) bounces the first message from every new address, which they do to cut down on spam. He has a record of bounced messages from "email@example.com", "firstname.lastname@example.org", etc. which their server bounced because every one came from a previously unknown sender. Yet, his Groups.io account shows no record of any bounces, and there is nothing in the Activity log about bounces from his address. Ever.
Sounds like a very user hostile way of doing spam prevention.