This comes from issue #79 (closed)

When using running element, since you're cloning the element, you're also cloning children, classes, ids and any other attributes. Which explains why it work that way. The properties you set in the @page @top-right are passed to the margin-box (to the parent of the element), and not on the element itself, therefor, you need to use !important to inherit the attribute from the margin box. Maybe we could see if we can target html element from the @page :

@page :first {
    @top-right 
        { p 
            { color: orange; } 
        } 
    }
} 

This would add the color: orange to all the p that go into the top-right of the first page so it will override the style from the body (as @media print doesn't by default).