MacBook Air to an external display has pink letterbox bars with fullscreen video

I’m not really sure how to diagnose whether this is a hardware or software issue. I have one of the new 2020 MacBook Airs and I’m outputting via an Anker USB-C to HDMI adapter to a TCL 6 Series 4K TV.

Everything looks good and works and I can output 4K @ 60Hz (took me a while to figure out I could only get 60Hz if I forced the TV to use HDMI 2.0 for that input instead of auto-detect, but I digress), but when I display video in full screen from the Apple TV app or Quicktime player and it’s letterboxed, the letterbox is pink instead of black and the rest of the image has a slight pink cast to it as well.

Moving the mouse to bring up the UI snaps the letterbox back to black and the discoloration of the image clears up, but as soon as I let the UI hide again it snaps right back to pink bars.

Anyone got any theories? I don’t even know how to test if this is a hardware or software issue or really where to begin troubleshooting at all.

Maybe the Apple is changing the rate when it’s playing pure video. Could you tell from the TV side if it switched framerate to 30 or 29.97 when it goes “pink”? The fact it works ok when the GUI is on, makes it sound like overlay is forced to 4k/60, but maybe it isn’t locked to that rate when it is video only.

Magenta (not pink) is an important ‘test’ color when developing software and graphics. Usually if you see it it indicates that something unintended happened, such as a bug.

I didn’t get a chance to mess around with my TV, but I’m in a vacation rental for the week and can confirm it happens on a totally different TV.

The TV info doesn’t tell me anything about if the frame rate is changing, as far as the TV reports it’s 3840x2160 / 60 UHD at all times.

Here’s a video I shot with my phone demonstrating: https://www.youtube.com/watch?v=0sRaPGrW8g8

My guess is that it is the adapter