After creating it with mkpart, are you formatting it with mkfs.btrfs? You need to both steps (create partition and format it). Also, try running partprobe or rebooting after making changes so that the kernel re-detects it.

There is a filesystem type field in the partition table. Formatting the partition won’t change it. Delete the partition and recreate it with the correct filesystem type. In parted you can do that with “mkpartfs”.