When someone is @mentioned in a ticket, the notification email puts their name in the text, but Microsoft doesn't treat it as a mention. There's no "@" icon in the message list, and the message doesn't appear under Outlook's "Mentioned" filter.
When a ticket email contains new @mentions, add the following header to it:
X-Mentions: person1@example.com,person2@example.com
In the body, optionally wrap the name as <a href="mailto:person1@example.com" class="mention">@Name</a> then it looks like a mention too.
This works for both the corporate Outlook version as consumer web version.