The current design of vSAN is that nodes in maintenance mode are still part of the cluster. CMMDS has no awareness of maintenance mode. So if the host in maintenance mode is the Primary/Backup that host is still the Primary/Backup of the cluster. Its resources will be unavailable to the cluster, but it will still be a member of the cluster.
When network upgrades/firmware changes/switch changes are done while this host is in maintenance mode, it can temporarily lead to packet drops, asymmetric connectivity, etc., which can lead to potential temporary network partitions in the cluster.