We call clean_text() on the notification message when exporting it for the template
https://github.com/moodle/moodle/blob/master/lib/classes/output/notification.php#L186

It makes impossible to use a notification with a button inside it.

Will be great to convert the admin notifications page (/admin/index.php) to use core_notification in the future but notifications there have links, help icons and buttons that are removed by clean_text