I actually intended to post this to Reddit but I thought I would contribute content to here instead to get the ball rolling here and do my part.
Anyway, this is a Windows XP-era machine I have at work for testing, and I had just this monitor plugged into it and saw the CPU fan trying to spin. I spun it a bit myself and it just kept going. I disconnected the HDMI cable and it stopped.
The monitor is actually DisplayPort, with a passive adapter to HDMI which then goes to the HDMI cable connected to this PC. The GPU is just PCI-E. The computer has some old ~2007 AMD CPU in it. The GPU actually doesn’t seem to work anyway, the PC posts normally but there’s no image from either the GPU or onboard, but when putting either another GPU or no GPU, there’s an image from the appropriate output.
HDMI has a 5V power rail (your adapter isn’t passive, it’s just parasitic on that rail). I’m guessing that this old hardware isn’t equipped with diodes everywhere, and the fan 5V is connected directly to the PCIe 5V rail, and the card is also not shielded and so the 5V rail ends up directly connected to the monitor.
If i was to guess I’d say that the adapter itself also has a part in this. I think it is sending back power when it shouldn’t.