That's normal and expected. CIFS is a session-oriented (STATEFUL) protocol and will be terminated if you go ahead with the Upgrade (specifically during storage failover). Therefore, it is recommended to inform end-users to end their sessions (save their work) before the upgrade. Usually, the upgrades are done under the change-approved maintenance window, which assumes the users are well informed and there is no questioning even if the sessions are disconnected. Simply go ahead and stop & start the CIFS server or end the CIFS sessions manually.
It is explained here in detailed:
https://kb.netapp.com/onprem/ontap/da/NAS/Does_clustered_Data_ONTAP_support_CIFS_SMB_NDO
Please Note: Only continuously-available SMB shares will stay up during Ontap upgrade.
https://kb.netapp.com/onprem/ontap/da/NAS/What_are_SMB_Continuously_Available_(CA)_Shares
