When running in command prompt (gpt disk with no partitions)
mkpart PartName ext4 1M 500M
partition is created and "print" command shows file system ext4
 
When running
parted /dev/sda mkpart PartName ext4 1M 500M
parted creates partition but "print" does show file system.
 
So "mkpart" works different in interactive mode and non-interactive (file system is ignored)