Mentioned users are not receiving email notifications

Issue Summary

we performed the workaround available in this bug:https://jira.atlassian.com/browse/ID-7089
The workaround changes the unmentionable user's username to something unique, which fixes the issue in ID-7089.
But now if the same user mentions other users, they are not getting any email notifications.

Steps to Reproduce

1. Step 1 Perform the workaround available in ID-7089.
2. Step 2 Remove the user (ex.A) from the instance.
3. Step 3 Invite the user "A" using cookie-based Endpoint and check if mentioning works.
4. Step 4 Login as the same user "A" and @mention other users

Expected Results

All @mentioned user should be notified.

Actual Results

Users are not getting any notification.

Workaround

Currently, there is no known workaround for this behavior. A workaround will be added here when available