Hi Corina ,
I don't know if there is an alternative answer to your question, but you can make sure with the help of microflow that only one user role is selected. You can add a before commit event handler to your Entity. I'm leaving a link below that can be an example of this. I hope it helps
https://academy.mendix.com/link/modules/182/lectures/1398/2.3.1-Creating-Entities-and-Attributes
Regards
Ridvan