In current WSO2 IS implementation (including 5.11.0), the post logout uri is validated against the callback uri that you have configured [1] and therefore, if your call back uri and post logout uri does not match, it will result in an error as mentioned above.

This concern is currently tracked with the git issue [2].

[1] - https://github.com/wso2-extensions/identity-inbound-auth-oauth/blob/5c3c468b6158dc4e4e82fdfe783be0d86d4cd556/components/org.wso2.carbon.identity.oidc.session/src/main/java/org/wso2/carbon/identity/oidc/session/servlet/OIDCLogoutServlet.java#L378-L382

[2] - https://github.com/wso2/product-is/issues/6397