Flash at bottom 1/5 of Screen - Force RGB Readback

I’ve been getting an issue over the last few weeks. We did just rebuild the system with Rocky but not sure it’s related.
I’ve been getting a horizontal flash on the bottom 1/5 of the screen. It would last only 1 frame and persist whenever I was on that particular frame. It might only happen on one frame of a spot, sometimes not at all. It’s not on the exported material only on the broadcast monitor. It does not appear on the flame UI.
I’m working from home and we use UltraGrid for broadcast playback remotely.
I went round and round with our IT guy and couldn’t find the problem.
I took a look at preferences. In the Broadcast Monitor preferences I toggled Force RGB Readback. Surprisingly that fixed the problem. In the manual it mentions it when getting errors using Paint.
Not sure why this works or if there is a problem in my system that it masks. I did find however if I blew up the scene or lifted the blacks, the error would go away. This makes me think there is something going on at the edge of the raster, not sure.