I purchased a used M6 Pro that did not come with the power adapter

i'm sure it's all about detecting how much current the USB port is being supplied with. QC compliant bricks have sufficient amps delivery so that's why it works. I have tested several non QC bricks, but are able to deliver at least 3A 5V, and they all worked with my M6. Let me also add that the cable quality is also part of the circuit power delivery, so good gauged cable is a must to let all the 3A of power reach the device unimpeded.