The server being temporarily removed from the replica set during a rolling index build is expected behaviour. The “Expecting replica set member, but found a STANDALONE” message is a topology-change log; no exception is thrown here.

We could adjust this log to WARN/INFO level (similar to https://github.com/mongodb/mongo-java-driver/blob/96d0c6baf86f3dc3fc3fae31b53efaefb9b87c28/driver-core/src/main/com/mongodb/internal/connection/AbstractMultiServerCluster.java#L262). (CC jeff.yemin@mongodb.com see original commit).