When an XRay instance has many users, the ones that have not yet been loaded in the "Assignee" drop-down list can not be selected from it

Short Description: In an XRay instance with many users and when searching for them in order to assign them to a Test Run, not all of them are able to be selected after being searched in the drop-down list. This happens because not all the users are loaded at the same time.

Steps to Reproduce:

1 - Make sure that there is an XRay instance with many users from A to Z

2 - Navigate to a Test Execution issue

3 - Pick a Test and and click to assign it

4 - In the Assignee drop-down list search for a user whose name starts with Z and try to selected it. Do the same for a user whose name starts with A

Actual Result: The user is able to search by the two names but only the name with A is able to be selected and not the name with Z

Expected Result: All users should be able to be selected