This is a common error even back in the Sysvinit days. /dev/disks/by-label is not guaranteed to map the name labels to disks on every boot, and it can change with boot timing based on the storage controller behavior. Use /dev/disks/by-uuid instead.
/dev/disks/by-label is a legacy from the old days of systems with static disks wired in a specific, unchanging configuration.