Disclaimer: Described below was caused mostly by wrong "Color space" setting.
Perhaps I didn't describe the problem correctly.
First, its not just gamma, its the iris itself who makes bright colors darker.
Second, PJ tries to restore original brightness by adjusting gamma. I can see how it works when switching the iris on and off manually when movie is paused.
It doesn't succeeds completely, bright colors are still dull.
Third, adjusted gamma squeezes dynamic color range. Bright details are lost.
Fourth, colors balance shifts slightly, not sure because of iris or gamma.
I don't know whether it can be fixed by calibrating gamma or not.
I found an old discussion on this problem https://www.avsforum.com/t/579460/gray-scale-the-dynamic-iris/0_100
I should say the things are not so bad. I've watched a whole film yesterday ("Largo Winch") with Cinema2. It's not a dark movie in general, but it has dark scenes from time to time. Also I watched "Moon" (2009) few days ago, with Cinema1 or 2, dont remember, with a lot of dark scenes.
Most of the time the issues described above was not distracting/annoying/obvious, even if I could say they are there.
Sometimes I was thinking "Wow its so dark!". But still, sometimes I saw that things are not right.
(Bright scenes look gorgeous btw. Punchy and contrasty. I was really amazed by how bloody black letterboxes look in bright scenes, considering the CR is only about 2000:1.)
So far I like dynamic iris more than I dislike its trade-offs.
The side effects get more obvious when I have to use subtitles. When I know exactly how bright colors should look. Letters are bright white in one scene and grey-muddy in next one. Not good. Perhaps I should develop my English listening skills. (Strange thing btw, I can understand 99% videos on youtube, but its gets much more difficult when watching movies).