This is a known SMB limitation:
sk111854 1100 ClusterXL does not fail-back to Primary member says that after a fail-over of the 1100 HA Cluster, when the Primary member is eligible again to resume handling the traffic, a fail back does not occur, and the former Secondary member continues handling the traffic instead. No fix is required; the system is functioning as designed.
So either leave it as it is - the nodes are in sync and it does not matter which one of both is active (that is different to GAiA Full Management HA), do a manual failover (the procedure from sk111854) or use the script from Dannys suggestion.
CCSP - CCSE / CCTE / CTPS / CCME / CCSM Elite / SMB Specialist