Why was a mention of an absent colleague not tagged?
Tagging is chosen per rule, and three limits then apply.
Only @-mentions in work item comments are tagged, and the tag appears once the work item view is reloaded rather than the moment the comment is posted.
Mentioning yourself is never tagged, because Jira sends the app no event for a self-mention.
Related pages
Mention Detection and Auto-Reply - where this is covered in full.
Notifications - more questions on this topic.