Hi all (mainly @Wayne),
I discovered that the
Set button in the brightness and contrast dialog adjusts the contrast equally when the
Propagate to all open images checkbox is activated only in grayscale images.
I tested this for example in the Boats, blobs and Bridges examples images .
But on RGB images (e.g. Clown, Lena and Leaf) the adjustment is only done on the active image, which I guess is not intended as such.