The QR code from http://buildbot.libretro.com/nightly/android/ should use black color instead of gray. Given that gray colors tend to change their brightness based on the viewing angle of the LCD, black color is better.
Look at how the QR code looks from my phone. My phone camera is slightly above the QR code, so it needs to be looking down. This is the natural position to scan such QR code. Compare the gray-ness of the QR code with the black rectangles and black text from the taskbar.
Now I’ve turned my phone upside down. The camera is now below the QR code, so it is now looking up. This is how it looks.
Alternatively, look at the photos at: https://imgur.com/a/uFIMT