How to show both first and last name of an user to basic users

Hi every one, 

I'm raising a question similar to this post  How to make other users' profile information (e.g., first name and last name) visible to a basic user on a report as it has not been updated since 6 years ago.

The issue I'm having is for this below code,when opening the UI with a system admin account I can see full user name while with a basic user account I can only see user's first name. 

forEach(local!userNames,
user(fv!item, "firstName") & " " & user(fv!item, "lastName"))

I tried the retrieveusersecurityrolemap() recommended in the old post but not sure what to do with the output of the function, because for both accounts I had the same output:  

Can anyone suggest me how to use this function or any other solution to show the same name for both account types? 

Thank you.