This has to do with access rights on System module level. A XPath constraint will be evaluated in the user access right context. And a user can only access it's own user role. And thus filtering in XPath on other roles is not possible.
While using a datasource microflow, the user entity access is bypassed by default and thus all roles can be evaluated.
But not sure for which function and role you need this feature. Using Sytem.User and user roles should be limited to the Application (backend master guru) administrator and not be used in ANY other user process other then authentication and validation of the user it self.