I've had providers with blackbox tokens. Basically a random string that has to be passed to the /userinfo endpoint to retrieve the user claims.

I've had providers with the id_token and access_token being the same value. Other where they were different values. One that didn't support refresh_token at all.

Then a variety of screw ups where the required claims in the JWT tokens are missing or incorrect.

It's wild really. It always takes a bit of work to integrate any OpenID Connect provider. Most libraries only care to handle Google or Facebook login.