userpicker browser does not return some users when filter by group

Issue summary

User picker does not list all the users in a group, when filtered by group. The users that are not listed however, can be shown if you filter it further with the user's username, for instance.

Steps to Reproduce

1. Searching for a group with users in it, for example "anothergroup" which has 13 users (all users are in internal directory),
2. Attempt to filter by group name in the user picker browser

Expected results

It returns all the users in the group.

Actual results

It only returns some of the users. This does not happen with the user browser in User Management.

Notes

• This cannot be reproduced in vanilla instance, see restricted comment: https://jira.atlassian.com/browse/JRA-63624?focusedCommentId=1002393&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-1002393
• When searching via the User Browser (i.e., admin page), the users found are filtered effectively at a DB level:

  SELECT lower_child_name FROM public.cwd_membership WHERE (directory_id =  '1' ) AND (lower_parent_name =  'anothergroup' ) AND (membership_type =  'GROUP_USER' )

  This correctly returns 13 users.

However, when searching for "anothergroup" via the user picker, here's the query that gets run on the DB:

SELECT user_name FROM public.cwd_user WHERE (active =  '1' ) AND (directory_id =  '1' ) ORDER BY lower_user_name LIMIT 1000

This returns all usernames in the database. The results are then filtered in memory. Thus, the bug seems to lies in the subsequent filtering that occurs within the user searcher in UserBrowserPicker class

Workaround

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