In AI 25.4, when GPU Performance is enabled, at some point when working on a file, or switching between screens, thin vertical and horizontal black lines will appear. They seem to flash on and off for a second or more as though Illustrator is switching between tabs or other open applications.

I've tried to take a screenshot/video when this happens but it doesn't show up, if you look at the screenshot/video, it's as though those lines were never there.

When GPU Performance is disabled, the issue goes away.

iMac (Retina 5K, 27-inch, 2019)
3.6 GHz 8-Core Intel Core i9
40 GB 2667 MHz DDR4
Radeon Pro 580X 8 GB

This is now also happening in Photoshop, and again, disappears when GPU Performance is disabled.