Certain users (normally non-itil) are unable to see certain columns on auto-complete for reference fields.
An example would be the email column for a reference field that references sys_user:
ITIL users would be seeing the column on auto-complete:
Versus non-itil users who aren't able to see it:
The user is failing read access to the particular column(s) that he/she is unable to see on auto-complete.
Check the corresponding field level read ACLs on the field(s) that the users can't see and make proper modifications to allow them access.