Duh, that was the issue.
It looks like the scope parameter isn't included in the request for the authorization url in the [node library](https://github.com/GluuFederation/oxd-node/blob/master/oxd-node/index.js#L166), however the [protocol documentation](https://oxd.gluu.org/docs/oxdserver/) indicates it should be allowed.
I'll probably make a PR for the node library and docs because I've noticed a few other inconsistencies.