For your future refence you can do what you want a little more straight-forwardly.
Set a Mask Shape keyframe like mentioned earlier, go down the timeline, double click a mask vertex (point, node, one of those little square dots on the mask) and you get a “free-transform” box around the mask(like in Photoshop). Rotate the “free-tranform box” and the Mask Shape will animate.
One Mask on one clip, the clip stays upright, the Mask rotates.