Azure Mobile App MSA Authentication Difficulties

So I'm working on a project to redeploy an application that was previously hosted as an Azure Mobile Service as an Azure Mobile App. Said application has a frontend (hosted as a Web App in Azure) and backend API (this is the Mobile App) that is called directly from the frontend's javascript. The upgrade had been going well up until I started working on the Microsoft Account Authentication piece. Following the instructions at...

https://docs.microsoft.com/en-us/azure/app-service-mobile/app-service-mobile-net-upgrading-from-mobile-services
https://docs.microsoft.com/en-us/azure/app-service-mobile/app-service-mobile-how-to-configure-microsoft-authentication

...got things mostly rolling. My application has been registered in the Microsoft Account Developer Center, I specified a Redirect URI following the 'https://{{my_backend_mobile_app_name}}.azurewebsites.net/.auth/login/microsoftaccount/callback' format, I configured the Microsoft Authentication/Authorization in the appropriate blade of my mobile app in Azure using the ClientId/Secret from the Microsoft Account Developer Center. Things go mostly well when I test it out: I can go to the frontend of my application and click on the 'Sign In' button, a new window opens with Microsoft's Login.Live site where I am prompted to login with a Microsoft account, I do so successfully and am redirected to the Redirect URL specified in the Microsoft Account Developer Center and it has Code and State values appended in the query string.

However, the page that is shown when I get redirected there is a 'You do not have permission to view this directory or page'. If I copy the URL, open a new tab, paste it in and hit enter it takes me to a 'You have successfully signed in' page. But neither way takes me back to the frontend of my application where it would log me in and continue into the site as everything is still happening in the new window that was opened for signing in. If I close that window, the authentication process sees that as a 'Cancelling' of the login and requires me to login again (which just repeats the above process). If I leave the window open and go back to the window with my frontend site, it is still waiting for a response from the login process.

So this leads me to my main question: How do I get the Microsoft Account Authentication process to actually return the login response to my frontend site rather than to the login window?

Side note, I've updated the SDK's for both the frontend and backend from the Mobile Service ones to the Mobile App's ones. In doing so I had to add in an OWIN middleware (per the instructions in link #1 above, under the 'Updating the server project' header) to my backend app. My current hypothesis is that this OWIN middleware isn't configured correctly for authentication and is getting in the way blocking the callback from working properly; thus, I am messing around with the app.UseAppServiceAuthentication(new AppServiceAuthenticationOptions()); piece.

According to your description, I assumed that you are using the JavaScript client library for Azure Mobile Apps. I checked this issue on my local side and found I could encounter the same issue as you mentioned as follows:

Then I tried to deploy it to azure web app (https://brucechen-mobile.azurewebsites.net/login.html), I found it could work as follows:

After searched the web I found a similar issue and refer to this blog about Fix the Ripple Issues. I log onto azure portal and choose my mobile app, then click "All Settings > CORS", add the URL for my local web application as follows: