Looks like your actress walks towards the camera in the video clip, which means she moves lower in the frame. Because you've initially lined up the shot with her earlier position, the video layer is intersecting with the floor plane. Then, as she walks closer to the real camera, and moves lower in the frame, she moves onto an area of that layer that is below the floor.
It's important to remember that while the video layer itself is in 3D space, it's still a flat plane. The girl isn't actually coming towards the virtual, 3D camera. She's just a video projected onto the flat layer.
To ensure that she's always composited on top of the floor plane, even if she's physically below it in 3D space, create a new Grade layer and place it between the girl layer and the floor layer. This will separate them into 2 distinct 3D spaces, so that the girl is always 'above' the floor regardless of where they both are in 3D space.
Does the video layer actually need to be in 3D? It looks like it would work just as well if you kept it in 2D, as there's no camera move.
Hope this helps!