Hi there,
Where I work we've recently found out what it looks to be a very strange issue (SPainter v. 2.5.3 ).
Long short story, the "normal" blending between layers that use a mask seems to be off, with painter heavily desaturating transparent pixels on bright backgrounds.
Please note that in Designer this blending mode ("copy") works as expected, exactly like in photoshop.
Here a picture showing the issue:

In the picture above, I've used the exact same color and the exact same mask in all the applications.
Please note that, if the background was perfectly black instead of white, the result would have been correct.
The more the background is bright, the more transparent pixels get desaturated.
I personally consider this a bug, not only because its behaviour is different from its twin app Substance Designer, but also because this is the most basic blending mode.
Hope this helps.
Cheers,
M.