Thanks for all that context.
Like I said in my reply.. email is hard. Others with more knowledge can weigh in, but personally I’d suggest not expecting so much of Discourse and email. It’s an online platform after all, and not a listserv. If you expect an email exchange with a handful of participants from/including an email address associated with a Discourse category to behave as though it were an email client, you will be disappointed.
If you are keen to dig deeper, you could collect and look at info from the affected emails, from an email client, including the email headers and email clients used (outlook is notorious for misbehaving), and see if you can glean something.