If I understand correctly, this plugin disables xpack security and takes over the user management of Elasticsearch? If that's the case, I think you might have to re-create the operator accounts using the plugin and give them exactly the same passwords as the operator generated passwords stored in [cluster]-es-elastic-user and [cluster]-es-internal-users secrets. Please note that this is not something we recommend you do in production and the behaviour could change without notice in the future.
I will raise an issue to discuss the possibility of allowing user-defined credentials in a future release.