I've just recently chatted to boukehaarsma23 about it, as it also affects me.

I have uploaded a video of what this looks like which you can see here:

https://www.youtube.com/watch?v=roHXWtHRhyQ

Interestingly the artifact lines appear to be aligned across the screens,
vsync? 

The main reason though I first came across this is by trying to manually
downclock my GPU. With 2x 60Hz HDMI 1080p monitors attached the GPU is
constantly in a higher power state (both mclkc and sclk) and wastes a
considerable amount of power at idle. This is not a problem with 1 monitor
attached.

Also of note is that even with 2 monitors attached this does not occur on
Windows where the GPU idles at 300MCLK/300SCLK and is significantly cooler.

For comparison about power, according to my UPS my system idles at 112-128W
when the GPU's are downclocking appropriately(1 monitor). This measurement
matches with Windows where the GPU is downclocking to state 0 appropriately.

When it's not downclocking appropriately (2x monitors) it idles at anywhere
between 160-176Watt.

Setup:
2x RX580 Gigabyte 8G (Monitors both on GPU0)
Linux 4.19.4 (Arch Main)
Latest stable KDE Plasma on X