MB-45468 during a rebalance the indexing service uses pools/default?uuid=<UUID> to check the status of the other nodes. The result contains information obtained from ns_doctor including the health of the node as well as the couchbase server version. When the node is unhealthy the server version wasn't available and so indexing failed the rebalance.
It would be nice if rebalance could check the "keepNodes" at the start and fail with a more informative message. e.g. rebalance failed - not all nodes are considered healthy.