Not so sure...
I think you forget the windfoils are not like sup and kitefoils, but they have the wing moved further forward, in order to use the "normal" tuttle box in many boards.
When I made my windfoil, using a kitefoil, I took a board and put the mastbox further forward.
But I had to move it even further forward later, where it was perfect trim now.
Remember, windsurf got some pressure down on the mast foot, thus you actually get the trim centre moved forward relative to your feet, compared to a kite or sup foil, and not rear like you say.
This is corrected by having more fuselage in front of the mast on windfoils (wing further forward), so trim is better and you can use the existing rear box.
Which btw is stupid IMO, as the leverage and forces get ridiculously high now, for no reason but compatibility with existing boards/boxes, which might not stand for it anyways
Also, for a SUP board you might want more board/planing area behind, in order to glide up on foil, when on a wave, so this is most likely why the masts are more forward also.