I just tried at OSC and it works for us. Here’s the configuration that we use in ood_portal.yml. Instead of redirecting to the IDP we redirect to the oidc_uri with the query parameter. We use OIDC too. So maybe it’s an issue of logging out the user on the apache instance, not the OIDC.
logout_redirect: "/oidc?logout=https%3A%2F%2F%{hiera('openondemand::servername')}"