Windows 10 Pro 20H2

I have a primary monitor (AOC U2790VQ) connected via DisplayPort and a TV connected as a secondary monitor (Sony X9500H) via HDMI. When both are connected and powered on all works fine. However, if I power off the TV, after a few minutes the primary monitor screen goes black for about 2 seconds, then comes back on and repeats the cycle every 5 seconds or so. This continues until I either disconnect the HDMI cable from the TV or power it back on. My GPU is an Nvidia RTX 3090.

Note that when the screen goes black, the power LED on the monitor remains fully lit - it's not going into standby mode (the LED would turn orange if this were the case).

Interestingly, even when the TV is powered off, the computer still sees it as connected - it still shows up in my Display settings as monitor 2, and the TV audio is still available as an output, so the TV must still advertise itself as a device via HDMI even in standby mode.

Things I have already tried in an attempt to fix the problem:

• I replaced the GPU (costly, I know but I was due for an upgrade anyway).
• Reinstalled Nvidia drivers with clean install.
• I bought brand new HDMI and DisplayPort cables and replaced for monitor and TV.
• I tried connecting the monitor via HDMI instead of DisplayPort.
• Changing power setting to never put the monitor to sleep.
• Disabling the audio output of the TV in Nvidia Control Panel under "Set Up Digital Audio" just in case it was the audio causing the issue.
• Reduced the TV Refresh Rate from 60Hz to 30Hz to minimize bandwidth usage on the HDMI cable.

All to no avail. All the solutions I've googled so far would have you believe the most likely culprit is a bad cable, but as I said I've already replaced all the cables and it's still happening. At this point I'm left scratching my head.

Any ideas would be most welcome. Thanks very much.