Service Upgrade Failure 4.1 -> 5.0.1 CE

I followed the process for the single node online upgrade. Removed one node at a time that was running 4.1 CE. Upgraded to 5.0.1 and re-balanced back into the cluster.

Upon removing the (final) 04 node running 4.1 I received this error during re-balance:

Service upgrade exited with reason {service_rebalance_failed,index,
get_agent,infinity]}}}}]}}}} hide

I re-balanced the 04 node back in after upgrading to 5.0.1 and the query/index services seem to work anyway. Why did I receive this error? Is there anything further I need to do in-order to upgrade the services to 5.0.1?