Actually x scale might need to be -1 as well
The best is to animate a simple rectangle and animate it so the speed is different at each frame and play with scale x and y +/- 1 so it matches – and try the 4 -1 or 1 x and -1 or 1 y… to see which one is the right one.
I know for example with the same mental ray renderer Softimage is to previous frame and Maya to next frame. And if you use MR production shader or LaMaison it’s different. LaMaison is scaled flipped -1,-1…
Pierre