I maybe found a bug:

There´s a layer - opacity for example 60%, black lines (looking grey because of the 60%). Above there´s a layer with 100% opacity, with black lines.
If that one is integrated in the layer below, the new layer has a opacity of 100%, and it is IMPOSSIBLE to change the "60%-lines" to dark black again!
The new lines of the layer from above are black...
The old lines stay gray "forever".