More details in link here:

If the RSM receives a ReplSetReconfig with new horizons shortly after responding to an isHello response, there seems to be no promise attached to the horizon in which to fulfill, and thus the ErrorCodes::SplitHorizonChange will never be sent to the node.

If the node never receives a SplitHorizonChange, it will not drop its current Streams to the RSM, and could be receiving isHello responses from a stale horizon.

The node should instead see that the Horizon is stale/changed, and drop its stream and re-establish an isHello stream to the new horizon.