Under some conditions, some specific users can't be mentioned in Jira issues

Issue Description

A user cannot be mentioned (=this user can't be found in the user picker list when typing its username), under the following conditions:

• the username of the impacted user has a dot (full stop) character in it
• there are at least other 10 users which full name or username contains part of the username of the impacted user
• these 10 users are ranked before the impacted user in the alphabetic order

This issue occurs with various fields such as the comment field or the description field, and in any screen (view issue, edit issue and create issue screens).

Steps to replicate

• Create 11 users using the full name and username listed below, and a dummy email address:

  Full Name	username
  User1 Totoro	someone1.totoro
  User2 Totoro	someone2.totoro
  User3 Totoro	someone3.totoro
  User4 Totoro	someone4.totoro
  User5 Totoro	someone5.totoro
  User6 Totoro	someone6.totoro
  User7 Totoro	someone7.totoro
  User8 Totoro	someone8.totoro
  User9 Totoro	someone9.totoro
  User10 Totoro	someone10.totoro
  User11 Totoro	totoro.tu

• Grant a Jira license to all these users
  
• Create a new project, and make sure that all these users have the Browse Projects permission in this project
• Create a new ticket in this project
• Try to add a new comment
• In the new comment, try to mention the user "totoro.tu" by typing @ and then "totoro.tu"

Expected behavior

The user "totoro.tu" should be listed in the user picker list.

Actual behavior

No user is listed:

The same behavior is observed with the description field:

Workaround

Workaround 1

After you looked for "totoro.tu" and did not find any user, click outside the comment box, and then click back inside the command box. The user will be magically listed:

Note that if you refresh the page, the effect of the workaround will disappear, and the workaround will need to be executed again for the user to be listed.

Workaround 2

Use the following syntax to mention the impacted user, instead of searching for it:

[~totoro.tu]

Workaround 3

Instead of search for "totoro.tu", search for "tu". If there are no other users that match "tu" and which are listed before "totoro.tu" in the alphabetic order, then the user "totoro.tu" will be found.

Note

A screen recording showing the behavior and the 1st workaround has been attached to this bug ticket:
Screen Recording 2021-09-24 at 10.07.37.mov