magnetic mask with overlayed clips produces too bright composit

I copy a clip and put it above the original. Then mask part of the top clip say the land in a landscape. So only the land is in the mask of the top clip. This makes the composited video too bright for the land.

I would expect to make the top clip of masked land 100% opacity with "normal" compositing. This, as I thought, would show either the top masked portion OR the bottom unmasked pixels. Thus resulting in a compoisted combined video the is no brighter than the original bottom clip.

However I have found no way to get rid of the extra brightness in the masked portion. If I set the top clip to opacity 0% the brightness, of course, goes awy but I can no longer use the masked part as I wanted since it is essentially invisible in the combined video.