I've tried 3.13, 3.11.0, 3.1.1 and in all cases, it makes a margin just for the first page which is expected.
Btw the page margin has this css
@page {
margin-top: 5cm;
}
Your problems could be caused by a different version of chrome which may apply some specific things differently. Unfortunately, there is nothing we can do about it. jsreport just uses the latest chrome as it is without tweaking it.