It is possible that you experience a small downtime due to a Live migration depending on what trigger the migration. The most common reason is a Host Error.
From the Live Migration documentation "Failed hardware such as memory, CPU, network interface cards, disks, power, and so on. This is done on a best-effort basis; if a hardware fails completely or otherwise prevents live migration, the VM crashes and restarts automatically and a hostError is logged."
If your service is really sensitive about this I recommend you to use a Managed Instance Group