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)