Zulip is an excellent user experience, and when deployed self-hosted is also quite secure. The only thing it doesn't have is federation, which would involve rethinking security.
This has been a problem ever since email: many people reply to the last mail they have from a list, sometimes changing the subject, and then get surprised when the mail is threaded under the mail they replied to.
I've seen this occasionally on Zulip instances, but a reminder and a suggestion that the topics are like email thread subjects or forum thread subjects tends to help.