Interesting, geezerhood.
The crop only appears when I click "new compare variant" - which should not include any settings at all.
Actually a new variant can have some settings automatically applied to it. One for sure in my installation that is set automatically when the file is imported and again if a new variant is created is the Lens Profile in the Lens Correction menu. Even if you click the Reset Adjustments on All Selected Variants icon for any given RAW variant, the Lens Correction / Lens Profile remains.
When I import my RX100M2 files C1Pro sets the RX100 II Lens Profile and selects Chromatic Aberration and Hide Distorted Areas, so a crop appears. When I shoot a Tamron 2.8 24-70 vc on my D800, it imports as generic since Phase One does not have a profile for this outstanding lens, and I never see any correction done automatically.
Perhaps when your original RAW was imported that setting was not applied for some reason, but when you create the Variant it was (Different Capture One settings before perhaps?).
Look in Lens Correction / Lens Profile and see what is there. Pull the Distortion Slider down to Zero or choose a Generic profile and see if the crop goes away on those variants. If it does, than that is the cause.
I am pretty new to C1Pro so I don't know yet how or if you can turn on / off the automatic lens correction profile application when importing images.
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Regards,
Don