Non-ITIL users cannot see any values in fields referencing the sys_user table and they would see 'No matches found' error. When they go to sys_user.LIST table directly, the users would see the user records.
Release or Environment
The sys_user table may have user records from the customer_contact table as well if the customer have customer service plugin activated. As a user without the customer service role, he should not be able to see the customer_contact table. That's is why it is not returning users in the reference field since he might not have access to the customer_contact records that are in the beginning of the sys_user list.
To resolve the issue, you can add a filter in the User variable to avoid the users from the customer_contact table(ex: Class is not Contact).