I think the reason you feel there's no mast foot pressure is because there really isn't any until you get up on the board, sheeted in some and hooked in.
Yes you can "steer" your board by pushing/pulling the rig to get it lined up correctly for a beach start (or a water start for that matter as it's really doing the same thing) but that's not true "mast foot pressure" as we speak of when you are up and planing, back in footstraps and hooked in.
So, my first (and perhaps best) guess is that you have somehow learned to position your board and push the rig up for a beach start and in this process you are depending on a little bit of "nose bite" (along the water line up forward from about the mast track and forward to stabilize the board's direction relative to the wind direction.
Unfortunately the Hypersonic is so short (and the nose is even shorter relative to the mast track position) that I think you probably aren't getting the same reaction from your board, so as you go to step up, it pulls under you a bit.
Also, as CC mentions, you may not be getting your foot near enough to the center of the board, so the board is responding to your foot being upwind of the centerline (the board is tipped slightly upwind rail down) by rounding up into the wind.
I'd suggest that you start off with the board pointing a little further off the wind. This will help until you adjust to the super short length and fairly wide width of the Hypersonic.
It's very different than your Carve, but I would think might have encountered some of the same issues on your Formula 135 ltr.
Hope this helps,