Yes I think it is the same problem mentionned in your second link.
I found a solution that seems to work Ok, though it eats a lot of ram : enabling "stroke attributes", putting the brush on "Normal" and lower the opacity.
here's a comparison between this setting and the classic "cover" and "soft" setting with the same default reseted digital airbrush :
The only problem of this setting is of course the lag when you work on another layer. So I've set my cursor on "iconic" and it reduces the lag considerably.
The thing that puzzles me is that they call this stroke attribute "Normal". So 1) They consider it a standard opacity mode and 2) they DO know how to make a brush behave exactly like a standard (sometimes dull, I agree) photoshop brush... I'm sure they want color to function in a weir additive color mode, because it makes color mixing a little more vibrant in the end. But I don't get why they don't want to leave a "Normal" mode on every tool. I'm sure this would convince a lot of people who use Photoshop by default only because they don't understand how color works in Painter...