I never had an Index so it is completely new for me.
I found that on Reddit:
"Pixel inversion is caused by a voltage issue of LCD panels. Your mainboard, drivers, CPU etc. are influencing this issue. Valve made a software "Column correction" just for that. Besides the technical influence, it depends on the user, too. How good is the personal eye sight, is the sweet spot found, mind blending it out etc. A friend of mine used my Valve Index and had hard times to see the pixel inversion albeit for me it was really clear to see. Good for you to not see this pattern. It is really distracting."
If it is a voltage issue, it might be both a hardware AND a software/firmware problem. I hope it is reversible.