IndiePass currently provides a URL to a HTML page as its client_id parameter.
The latest IndieAuth spec recommends making this URL point to a JSON file instead.
See: indieweb/indieauth#133 and https://indieauth.spec.indieweb.org/#client-information-discovery
Hopefully this should be as simple as providing a suitable JSON file on https://indiepass.app/ and changing the client ID to point to it.