I have seen this behavior and could not pinpoint the cause, but assigning a homepage to each userrole seems to prevent this from happening.
Btw, if you want this role-switching option also in your -test and -accp and other environments, try the Base application.
I assume you have used the login page as the sign-in page (authentication) and the Home page as the default/role based home page?
If not: worth checking :)