Comment 4 for bug 1851438

https://marc.info/?l=util-linux-ng&m=142920033601419&w=2

Seems to say this is by-design.

util-linux/fdisk versions in Xenial and newer all have this issue.

The workaround for user-data is to specify:

disk_setup:
  <disk name>:
     layout: [100]

Which will create a single partition spanning the whole disk.

And cloud-init will need to emit ",," as input to sfdisk moving forward.