Mentioning users in the issue description on issue creation does not send out notifications

Issue Summary
While creating an issue, and you mention a user in the issue's description, a notification for the mention is not sent out. The notification will only be sent out if you mention the user after the issue is created, but not during issue creation.

In this screenshot Notification2.jpg, the notification is sent only after I have mentioned the user after the issue has been created. The first mention was not sent out.

Steps to Replicate

1. Create an issue in a JIRA instance that has outgoing mail configured.
2. In the description field while creating the issue, mention a user using the "@mention" function.
3. Create the issue.

Expected Result
JIRA would send out a notification for issue creation as well as one to the user mentioned in the description.

Actual Result
JIRA sends out only a notification to confirm issue creation.