I've noticed that when a user on the desktop client creates a ticket, the "Assign To" field should be a combo box but it's just a regular text box, no values work in it.
But if the same user goes on the web, the dropdown for Assign To is there and works.
This is NOT the same for Administrators
They see the combo box in both desktop and web.
By default the "Assigned To" attribute has the "members only" option enabled, which means that it will show no list of users when the project in which you're adding the issue doesn't have any members. So this on the project, not on the client application or the user role. You can switch off the "members only" setting for the "Assigned To" attribute if you want all users to be shown regardless of project membership.
Regards,
Michał