The latest docker image available in the dropdown of Managed controllers is not the last version available for CloudBees Jenkins Enterprise.
When you upgrade CJE, it adds the new Docker Images to your CJE environment to upgrade your Managed controller, only on exceptional occasions. For example, if we want jobs configured on one controller to make use of new CBJ version, when it is detected a security issue, or we want to resolve an urgent bug, in that cases you could add Manually the new images to be possible to upgrade Managed controllers without upgrade CJE.
You can add new Manage controller Docker Images on Manage Jenkins/Configure System, then go to Docker images and add a new one
You could see the version of the Docker Image on https://hub.docker.com/r/cloudbees/cje-mm/tags/