This time error showing a negative back step is only a few pixels. I would think this is due to time problem between the speed of the screen video timing and the main time base are out of sync by a few pixels.
Could be a component delay or a small error in the Dac division..
If its real you have found a way to travel back in time
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which is really good .
I think you are pushing your scope .
And with all electronics there will be one part that may be not 100% up to spec.
I would check your video memory maybe getting hot..
Have fun
RNS