User picker "in group" filter does not display all users when having more than 1.000 users in JIRA.

Summary

User picker "in group" filter does not display all users when having more than 1.000 users on JIRA.

Steps to Reproduce

1. Deploy a clean 7.2.6 instance;
2. Use the JIRA Data Generator to create 1.100 random users;
3. Manually create 5 users and add them to a test-user group;
4. Open the "User Picker", we'll notice that not all users from JIRA are picked as per message "Displaying users 1 to 20 of 1,000" on the screenshot below;
   
5. On the "In Group" field, select the "test-user" group and hit the "Filter" button;

Expected Results

• All 5 users added to the test-group to be displayed;
  

Actual Results

• Not all users are displayed;
  

Workaround

Use the user management view to filter the users from the group instead using the user picker.