Email Notification for User Mention is not Sent when 'Notify watchers' Is Not Checked

After mentioning a user and saving a page, the mentioned user will receive email notification. However, when 'Notify watchers' checkbox is unchecked, the email is not sent to the mentioned user.

Steps to replicate

1. Configure Mail Server
2. Edit an existing Confluence page
3. Mention a user in the page
4. Before saving the page, uncheck the 'Notify watchers' checkbox
5. Save the page