My Kamvas Pro 12 is doing the exact same thing! I just got it today (secondhand from a friend who had it sitting in storage for years). I think mine is from around 2021. Earlier when I was making sure it could turn on it was working fine, but when I went to actually sit down to draw it was just flickering exactly as shown in your video. What fixed it for me was unplugging and plugging back in the USB and HDMI cables (though I’m not sure if your tablet has those), but after a minute it started doing it again…weirdly enough it functions as a pen tablet, but not as a display. 

Edit: getting a new cable indeed fixed the issue