Okay, this is a viewpoint. Thank you.
Please bear in mind that the problem you have experienced does not affect ALL users. Not all RX560 GPUs (the title of your thread) behave this way.
A known problem - just like yours - is that
earlier motherboard BIOS files had bugs affecting GPU fan speeds, which Gigabyte addressed with an upgrade. It is true that the latest motherboards should already have a newer BIOS.
I specifically mention F4 and F5 because I have experience of this, and if you check
Gigabyte Support BIOS files you can see F5 addressed GPU compatibility.
As for macOS being responsible, you say
AGPMInjector does not seem to have helped much.
Is there one particular brand of RX560 that requires a special cable to fool the motherboard?
You mention that the problem persisted, although not quite so badly, with your RX470.
Why would this not happen under Windows?
Clearly cabling would be an 'electrical' issue and not a software one.