One data point, since I have seen this issue:
In Thunderbird (I’m running v78.7), I believe that it recognizes Discourse emails as coming from a mailing list and offers two reply buttons – Reply and Reply List.
Reply will create a reply going back to the “From” address (noreply@xyz.com)
Reply List will create a reply going back to the “Reply To” address (replies+01234@xyz.com)
I personally find it very easy to accidently hit the reply button instead of the reply list button.