Thanks, this is awesomely informative. Yes, that seems to be what's happening. It's actually amazing that it hasn't been happening more often. So far, I've noticed it only with this one group member, and she (thankfully) is extremely cooperative and willing to be careful to always use "reply all."
As for the groups.io change that you say could be made to rectify this ("An account setting could be established to rewrite this From: address to match the group address, but only for those groups that have both "Reply to Group" and "Remove other reply options" set), I think that might cause more problems than it solves, and/or I don't understand it. It could be done in my group, which has both of those options set. But would that cause emails from this particular person to not have her correct "from" address in emails people receive for her messages to the group? I think I'm getting "from" and "to" confused in the fix.
Messages are the sole opinion of the author, especially the fishy ones.