This is checkable on any User Picker in Jira that uses https://instance.atlassian.net/secure/popups/UserPickerBrowser.jspa. (i.e: the one you see in the "shared filters" page).

The picker seems to only actually search in the system when using the "E-mail contains or Full name Contains". When doing so it brings a maximum of 100 users (let's call this initial search). Filtering for groups after the initial search will only bring results from within the initial 100 users.

If you re-do the search using the text searchers you re-do the initial search creating another array of 100 users in which the group filter will be limited to.

Steps to reproduce

1. Create a user starting with 'B' and another with 'A'.
2. Insert both users in the same group
3. Using the picker search "Full Name Contains" = A > Click filter (feel free to erase the search box after that, it won't make a difference)
4. Use the group picker to search for the group where those users are in.

Expected results

It is expected that a subsequent search meets the conditions that you're searching for, not cascading results from the last search, this can cause confusion as users may think that users are not being returned.

Actual Results

Only users that were on the search for "A" will be found (the user with the letter B won't be listed).

Workaround

Use always an alphanumerical search before filtering for groups.