At the wiki page Running FoundationDB in the Cloud · apple/foundationdb Wiki · GitHub the Ephemeral SSDs section says:
In theory however, you would lose all your data if a whole region would go down due to a power failure.
For the instances with attached nvme drives, power failure does not lose data. Data is only lost when you stop or terminate a node, but any crashes, reboots, or other non-machine-releasing events maintain the same disk because the machine is not potentially remapped to another host.
The data on an instance store volume persists even if the instance is rebooted. However, the data does not persist if the instance is stopped, hibernated, or terminated.
See https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/InstanceStorage.html