I'll take a stab at it.
What Firmware are you on? 22.214.171.124
Firmware seems to have fixed some
Long ago, when I first tried to setup my 4K Roku, I saw goofy video
patterns. So I setup the Roku as 1080i... not 4K. That worked.
So, after many months of reading messages about how critical it is to
have a quality, working HDMI Cable for 4K (18Gbps), I realize I'm going
to have to swap-out that cable to the one that's on my Sony DVR - which
I think is the Vizio Cable. (Can ya tell I haven't used my Roku in a while?)
Make sure your supplied Vizio Cable is hooked-up to the highest priority
HDMI device. Perhaps your Roku.
You could unhook all HDMI devices and do a Soft or Factory Reset to see if
you problem clears-up when Tablet-Casting.
Then, one-by-one... hook up one HDMI Cable to one device to test for a while.
Then hook it up to all remaining devices. If that Cable passes all your devices
in testing, repeat if you have other Cables to test.
Possibly could be a Vizio hardware problem.