Some oopsies come to mind: accidentally turning on alpha add for a layer beneath the solid, accidentally assigning the layer above as a track matte or accidentally creating an unwanted mask on the solid.
It doesn't sound like you're animating the solid's position or anchor point as you rotoscipe, which could definitely do what you describe.
Well, we just ended up changing the solid size, and change the position of our mask to line up with our object. And that seemed to work fine... So far. Thank you for replying.