After applying Windows Server 2016 September's patch (KB4038782) to a node in an S2D cluster, the disks on that node would not come out of maintenance mode after the node re-joined the cluster and after Resume -> Fail Roles Back. The VMs would move back but the disks would stay in maintenance mode, thus causing the virtual disks to show a status of degraded. I had to manually take the disks out of maintenance mode after the node joined the cluster and after I failed the roles back.
To see if the disks are in maintenance mode run:
Under the OperationalStatus column it will say "In Maintenance Mode" by the disks for the node that was just restarted. I don't know if this issues was/is just specific to me or if it may happen to everyone that applies the patch. To take the nodes out of maintenance mode use the Disable-StorageMaintenanceMode command.
I have a smaple here that gets all the disks in maintenance mode and disables maintenance mode for those disks.
Update: Official MS KB on the issue is here