You don’t show your exact code (just one line when obviously there must be more), and you don’t say what it does and what you expected it to do. You also don’t say what is the situation when you run the code.
Therefore it’s difficult to say whether you have encountered a bug or whether you are just expecting different behavior than Notes provides.
Also, a point of terminology. You don’t print a form. You print a document using a form. I assume you want to print the same document with its original form and then with diferent forms.
You might note from the help for FilePrint command, that the form argument is ignored when command is used from a document window.
You can use ViewSwitchForm to open the document using a different form, and then print from there. You’ll have to use @PostedCommand for all these commands, since they will otherwise execute out of order. As I just pointed out in another message, it’s also possible to use a dialogbox containing a self-printing form, and just open that dialog.