Row Level Locking Issue - Working for some users and failing for others

Hi, 

I've previously set up RLS successfully for my PBI reports. The company has had a revamp so reports have been re-created.

Just like before, I've set up RLS in my Power BI report and created 2 roles like so in PowerBI Desktop, then popped onto the Security settings on the app.PowerBI and added the usernames.

UserPrincipalName()=Concatenate([USERNAME],"@company.com") 

Role A has been tested, it has 21 users and when I view as, I see that chosen user's data.

Role B is failing, it has 114 users and for some reason, it works for 2 users randomly. Some observations:

• Testing those 2 users that work, all visuals spin and refresh to the user, takes a second or so. 
• Testing other users, sometimes only 1 visual spins and nothing is filtered to the user. Other times, all visuals spin for a longer time and once again nothing is filtered.

The users will access the reports through the app so I believe do not need to be set up as users (viewer role) in the Workplace.

Please can anyone provide any advice on what's going on? Let me know if you need more details?

Thanks!