The effect you mention, has bugged me since around 4.1x. The older 3.7.8 did not do this. Mid 2010 I had a open tech call for months
the Phase One on this same issue. Net many times the image looks OK on the LCD, however that same image will load in Capture One
with about 1 to 1.5x of exposure difference on the plus. You can see this as you mention, as the images first load in Capture One, small
they seem to have the same exposure level as they did with the LCD but as Capture One creates the cache file of each image, many times
you can see the image appear to "brighten". For blue skies this created a real issue for me and for a while I used the Linear setting instead
of the film standard. However I didn't like the look of these files.
For me the solution was to watch the histogram and under expose the the shots just a bit. I never have figured out what causes this, but
just changed up my style of shooting. It helps to under expose a bit any way with a P45+ as it tended to blow out high lights too easily for me.
This is using a P45+ with a DF body and a variety of Mamiya lenses, 35mm, 45mm, 55mm, 120mm 150mm 75-150mm. Window7 64 bit, Capture One
4.x up to 6.2.x 64 bit.