In my experience in VSAN 6.0-6.2, the following will happen if a copy disappears for 60 minutes and then returns later:
example: VM with FTT=1, 1TB VMDK
a host is left in maintenance mode for 60 minutes and components for this VM go absent
resync occurs at the 60 min mark, lose 1TB of disk space as the new copy is resynced across the cluster
maintenance mode host, exits maintenance mode and rejoins VSAN cluster
the resync across the cluster will take time to finish and then shortly after, the copy of the data from the maintenance mode host (that has exited maintenance mode by this time) will be deleted.
In this scenario, for a short period you will end up with 3 copies of the data, this can be problematic if the VMDKs are very large or you are close to running out of space. They mentioned that they may change this behavior in the future but it hasn't happened yet to reduce rebuild times from events like this, but I haven't seen it happen yet.