Right, I think I have solved the problem.
My MacBook runs at 60hz. My adapter was an Anker 7-in-2 device which only allowed 30hz output via the HDMI port. I have tested a separate USB-C to HDMI dongle and the problem seems to be entirely resolved.
I played videos in full screen, used the camera on my MacBook, and still no issues!
I have now purchased an Anker 8-in-1 adapter, which only takes up 1 USB-C port, and also allows 4k @60hz via the HDMI port.
I am a skeptic so I must say this: even if this for some reason doesn't work using the HDMI directly into this new dongle, I will still have a spare USB-C port on the MacBook itself, so I can use the USB-C to HDMI dongle I am currently using, which I know does not experience the problem.
Let me share my entire setup details in the hopes that this is of any use to you guys.
- 2020 M1 MacBook Pro 13"
- Acer VG272UV monitor
- Standard HDMI cable which came with the monitor
- Cheap USB-C to HDMI dongle from Amazon (make sure this supports your resolution and 60hz) -> I will be swapping this out for the new multiport one I ordered from Amazon, via Anker, when it arrives tomorrow.
Additional info:
I have gone through all the troubleshooting steps with Apple support, deleting various files from the library, trying safe mode etc. While I believe that the problem for me was entirely caused by the incompatible HDMI port on my adapter, I believe for transparency's sake I must mention this additional info.
I sincerely hope this is helpful to anyone, this problem has been infuriating, but I am relieved that in my situation it seems to be a relatively simple fix.
If for any reason the issue persists (I'm begging it doesn't ahaha), I will come back to update this thread.