Authorization in web app with web forms and MVC

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My team is converting a web forms application to MVC (and switching to C# from VB.NET). The reason for this is that we want to roll out changes to a module at a time, continuing to use old pages until all is replaced. We have a custom role provider defined. In the new code, everything works perfectly. However, in the old code, if we attempt to navigate to a page that is in a folder with a web.config file that defines authorization rules, the role provider is never called. Has anyone encountered this? If so, have you solved this?

What I have tried:

We removed those "child" web.config files after which point the old pages can be navigated to (of course, we need the authorization rules, so this was just a test to determine if they were part of the problem). We also tried moving the authorization rules to the parent web.config via <path> elements, but the same thing occurs.