We're working on a fix for this. As a workaround users can either test out roles using the Power BI Desktop or can continue to use the Power BI service by using their browser's back button to go back to the security page and test a different role.
You can track the latest resolution status by vistiing our support page at https://powerbi.microsoft.com/en-us/support/.
I just tried. It works now. You can have a try.