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I had this happen to me too on just one occasion. But instead of red I had blue blocks. Like you, a reboot took care of it and it has not returned. This happened to me on the first variant of 5.12 I had installed at the Marietta Service Center on 1 July.
The red blockiness happened to me on 5.12 (1.64.26 I think) this past weekend. And generally I've had to reboot every week or so lately. I'm really looking forward to 6.0...
I think the culprit is the browser depending on the web sites visited that requires rebooting from time to time. In my case the voice recognition turns flaky first.
That exact thing happened to me yesterday. I was exiting a parking structure into brilliant sunshine and the screen did that at the same time the auto headlights turned off. Weird.
Like me everyone else here, I did a screen reboot which cleared it. I'm still on 5.11.