If you're of certain age, you'd have long heard about the Z-20, or more precisely, their effort to build a local version of the S-70 blackhawk helicopter, and the project's apparent "death" because of the technological gap and Chinese have imported so many Mi-17/171 to fill the role.
And now it finally have its first prototype first flew? There are plenty of eyes trained onto the Z-20...sure it ain't as glamorous as fighter jet but to the not-so-infantile amongst us, we understand the underlying significance should this project reach production stage.
The Y-20 is, I admit, another kind of animal, because it was earmarked in the 5-year plan.
Helicopters generally don't get as much coverage as jet fighters or other planes.
Simply remember how long it took for us to get decent pictures of Z-10, or how long it took for us to get a new picture of the Z-8YJ.
Y-20 was completely different, not only because it was in the 5 year plan, but also because of the greater prestige and technological difficulty and exclusivity of developing such an aircraft. Indeed, I doubt we'll ever have the kind of quick CCTV coverage which we did when the first y-20 prototype flew last year, ever again.
And like you said, Z-20 is definitely less glamorous and is sort of the "bastard child" -- however helicopters also generally get less coverage by PLA watchers anyway. They are also easier to hide.
so all in all, I don't find the lack of Z-20 pictures uncanny at all and fits with previous precedents.
And I don't think there's any reason to think the first flight was "alleged".