Preview app not displaying PDF form fields... is this normal?

Hi,

Are there areas in which Mac OS and iOS and iPadOS's Preview app won't fully or properly display fillable form elements in PDF's?

I have created a PDF form with PDFPen that has a Signature field and several text entry fields. When I save them and then open them with Catalina's Preview app (or Files in iOS and iPadOS) the Signature fields don't appear at all.

Is this a limitation of Preview app or a problem with PDFPen. By the way, Adobe Reader seems to display the forms just fine.

Thanks!!