With v3.5, Zookeeper supports dynamic reconfig. Solr will happily connect to an ensemble with dynamic reconfiguration enabled, but it is not "aware of it". The ZK Status screen does not display whether the ensemble is in reconfig mode and is not able to load the dynamic set of hosts from zk.
This JIRA aims to make the ZookeeperStatusHandler able to display zk status as well as show status for all zk hosts in the ensemble, even ones that are not part of the static connection string that Solr is aware of. It will also show warnings if Solr's static zk connection string is out of sync with the truth.
Currently Solr has a static ZK connection string and is not able to re-configure itself when zk cluster is reconfigured. Fixing this is not part of this issue but is deferred to SOLR-14389.
I discovered this while experimenting with the solr-operator for Kubernetes. It configures a reconfigurable ZK ensemble and configures a single LB host:port as its zk connection string. This issue will flag a warning in this case since the connection string is different from the set of zk hosts in zk node /zookeeper/config. This warning can be removed once Solr fully supports dynamic reconfiguration.