Hey, I think I'm impacted by the same issue.
I've replaced the original fan of the HC4 with a Noctua NF-A4x10 5V PWM earlier this year.
It was working as expected, but since couple weeks the fan is at 100% speed continuously, which is loud.
It sounds like it's a software upgrade that broke the PWM control, but no idea which one.
I've done part of the same testing you mentioned but didn't managed to find a fix neither.
I'm probably even more beginner than you, so I don't think I can help much unfortunately.
How can we get more eyes on this issue? Did you post on Odroid Forum?
Georges Holland got a reaction from Adrienne in HC4 PWM Fan Control Not Working - Need Guidance to Contribute a Fix