after reading a thread here from september, I did the same test and found the same bug on amf codec:
changing CQP number (0-51) doesn't change bitrate.
did same test on x265 but here it works perfectly, as I expected
I did tests also between "speed" and "quality" settings, always in amd amf hevc: they have the same speed. I think it should be very different ("quality" seems too fast for me, it's even faster than "ultra fast" of x265)