Make sure you are using the right authorization server, the one you created the custom claim for, as the issuer of the token.
It sounds like you are using the Org Authorization Server (where issuer = Okta domain), but you will need to update this so that you use the Custom Authorization Server with this claim to get tokens. If you go back to where you created the claim and then switch to the “Settings” tab in the Admin Console, you’ll see the Issuer URL for the server.