Using an underscore ("_") in a redirect URL for an oauth flow returns a "/?error=flow+not+supported". Example: Registered a redirect url of {baseURL}/oauth/fortellis_redirect, and received the aforementioned error after authenticating. Changing the redirect url to {baseURL}/oauth/fortellisredirect will properly return the PKCE code from the flow for the user. Underscores are valid URLs and the flow should be able to handle them.