The replacement is done on the first user that matches or is a partial match. I have to redesign it to work better as it currently is just using a wildcard in its query so you can end up with users matching that shouldn't. You can use @USER_ID or @USERNAME for the most accurate matching, but @NAME can be inaccurate as it's not unique. Usernames that contain the value of Name can also cause weird issues.

And finally if the username is numerical or an email address the tag process doesn't link to the member at all as in it doesn't become a link.

Numerical @MENTION usages are treated as user id. Email address already contains @ so it can't correctly match. Will improve it to also check username instead of only user id if it's numerical.

It's not going to work with every usage in existence. It's designed to function with Joomla standard requirements. If you use usages beyond that it may not match properly. This will be improved later when it provides an ajax auto-complete dropdown to specifically select who you're wanting to mention. Next release will also improve the query to get rid of the wildcard, but until the ajax auto-complete usage it's going to have some inaccuracies depending on your usage.

forge.joomlapolis.com/issues/5419