Absolutely a no-brainer: when adding a custom domain name to a PowerApps Portal, that custom domain is not automatically added to the Azure AD redirect URI's.

This causes the Azure AD login feature on a PowerApps portal to be broken when accessing the portal through the custom domain (instead of through the default *.powerappsportals.com domain): it gives the error "AADSTS50011: the reply URL specified in the request does not match the reply URLS configured for the application [applicationID]"...

It should be a small change in the behind-the-scenes flow for adding a custom domain name to a portal, but would save a lot of time (and headaches) for people (like me) who are experiencing this problem.