Ok, there was specific problem affecting constant infinite lights (sky and infinite lights were working fine).
This is before the bug fix (16 samples/pixel):
and this after the bug fix:
What surprised me, after the fix, is how useful can still be Env. Light Visibility Cache even on glossy-nearly-specular surfaces:
So I'm probably going to remove the glossiness threshold parameter altogether. You can already achieve the same result by setting the value to 0.0.
Now I will go back checking the original test scene.