@Raki I’ve just tested your settings and it seems that your Role Mapper is incorrectly set.
You don’t have Multivalued enabled. Roles are sent as an array in JWT token.
i.e.
Multivalued must be enabled even when you send only single role.
That’s why you have square brackets in the roles view in OpenSearch Dashboards UI.
Also, you should use kibanauser instead of kibana_user for the built-in backend role.