Is it possible that a sub-par HDMI cable could cause issues like this?

Could be a bad HDMI cable yes.

But how you describe it, I don’t know, but they seem to be artifacts which also could be that your GPU is dying.

Have you tried it in Windows 10 yet?

If there was a problem with the cable, then you wouldn’t get an image. I don’t think HDMI 2.0 does link training so it doesn’t detect bad cables and force lower link rate like DisplayPort or Thunderbolt or PCIe or HDMI 2.1 does.

For 4K 60Hz, you need to use 8bpc to use 444.

In macOS BigSur, for the CAC-1080, macOS chooses 420 for HDMI timing 594MHz but it will choose 444 when using CVT-RB timing 533 MHz. It’s annoying because macOS doesn’t let you change the bpc (except if you can choose HDR then you know you are using 10 bpc). One can create an EDID override (at least on Intel Macs) to remove the 420 option but then HDR wouldn’t be possible. I haven’t been able to use HDR with the CAC-1080 – but I can with a firmware updated CAC-1085 (my CAC-1080 already has the latest firmware update). I can use the AGDCDiagnose command on an Intel Mac to get output pixel format and timing info.

In Ubuntu, the xrandr --verbose command shows the current timing but I”m not sure how to see the output pixel format. It seems to me that Linux doesn’t normally support 10bpc so there’s no reason for it to choose 4:2:0. In my test, using 60Hz 133kHz 533MHz or 60Hz 135kHz 594MHz the chroma subsampling test image was clear. Which Linux are you using? What GPU?

In Windows, there’s an option to change the bpc and the pixel format. For 4K60Hz, I can choose:
4:4:4/RGB 8bpc
4:2:2 8, 10, 12 bpc
4:2:0 8, 10, 12 bpc
but sometimes the display goes black or shows static or has the wrong colors (I’m using AMD RX 580 with the CAC-1080 – Radeon Software is garbage – sometimes the Displays tab in Radeon Software Settings disappears). Windows has an HDR option but I can’t get it to work properly (Radeon Software problems?). With a CAC-1085, I can get HDR but only at 30Hz – the Radeon Software won’t let me choose 4K 60Hz 10bpc 4:2:0 HDR at the same time.

I’ve upgraded to a GTX 980 and still can’t get the chroma test looking right even with the native HDMI 2.0 port. Xrandr shows 594MHz/135KHz. Running Ubuntu 20.04 LTS.

Some updates from my last post.

CAC-1080 has a firmware update that allows HDR in macOS.
https://forums.macrumors.com/threads/thunderbolt-hub-with-ethernet-for-big-data-transfers-recommendation.2278473/

The Nvidia drivers in Linux have an option to change the chroma sub sampling. I guess you’ve already seen that with the 460.91.03 driver?