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.
You've confused by-label with the kernel's device nodes (e.g. /dev/sda1). Kernel names are assigned in order. Labels are just like UUIDs and part of the filesystem.
However, unlike UUIDs they don't necessarily exist, are not as likely to be unique (you should only get duplicate UUIDs by cloning filesystems) and are easier to change. This makes them less suitable for mount configuration.
/dev/disks/by-label is a legacy from the old days of systems with static disks wired in a specific, unchanging configuration.