I wonder if there a difference between these two scenarios when I expand vSAN.
1.add new host in vSan cluster when host is in maintenance mode
2. host is not in maintenance mode
The only difference is that if a host is not in MM and HA/DRS is enabled it may have compute workloads migrated to it.
If it has Disk-Groups configured then it will also become an available space for data-component placement if out of MM but it won't automatically move data here unless you have any capacity drives >80% used (with default settings) or any new Objects (e.g. vmdk, vswp) may be placed here.
6.7 clusters won't allow moving hosts in/out of vSphere clusters without them being in MM but there are of course ways around this but that may have consequences with regard to vDS etc. (e.g. disconnect, remove from inventory, add host to inventory in cluster).