Ditto on a recently purchased Q90T-75". Logged a warranty fault with Samsung, 3rd party engineer came and took pics of the issue (he agreed it was a problem) and 2 weeks later Samsung closed the repair stating the issue is expected behavior on the screen. What smells here is I also bought a Q80T-65" at the same time and this has no rainbow effect at all. Very annoyed and frustrated after spending £4K on Samsung TV's which are supposedly the best, clearly not !!
Same here, loved my first Samsung 65’ purchased after Sony failed on me, so few years after, I have decided to invested $ and go with the Q90 85’. TV is great in the day time, but at night when turning on lights… will go with LG on my next purchase