White-space is very finicky in SSRS and you are experiencing this different behavior because you are using two different renderers and they are just flat out behaving a bit different. Looking at your design screen, it appears that you have just floating textboxes and white-space underneath between the last content and the footer. HTML rendering can be tricked into rendering that white-space before the footer by placing empty content (textbox or rectangle) in that space like so:

Red box: I would try inserting a blank rectangle off to the side, but the whole height, first.
Blue box: The next option would be to wrap the whole page contents into a rectangle.
It's actually a bit funny because most people are stuck trying to get rid of white-space in their HTML rendered report and you actually have the opposite issue here.