I am in the same position.
My app is live on the AppStore (for roughly 12 hours now), "A Repeatable List", however:
the app clip banner does not appear on the website (https://www .atrinh.com/list/index.html)
scanning an NFC tag to the website shows the App Clip card image as expected, but the lower half says "App Clip Unavailable"
scanning a QR code to the website shows the App Clip notification, tapping the notification shows the same "App Clip Unavailable" experience as NFC
sending the URL in iMessage appears as a link instead of an App Clip prompt
I am convinced that it might be due to deactivated Advanced App Clip Experiences I have added during development and testing using different domains and websites. (There does not appear to be a way to remove these completely).
I currently have 1 active experience that is producing the results described above, and 2 deactivated ones pointing to different domains & websites.
The App Clip was completely fine during TestFlight testing (TEST/OPEN from TestFlight), however the only problem appears to be the experience in production.
To see what the App Clip experiences should be, you can test the following URLs:
https:// unwind.to
https:// chibi.app
EDIT: Extra information, when testing on a new device, the App Clip appears with the correct title/subtitle/button text, however once the image loads, the content is reloaded showing the "App Clip Unavailable" message on the lower half with the correct image on the top half.