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I don't know the name of the setting, but there is some override setting that does change aperture. It might be related to ISO override or something. Search for the word override in the e-manual PDF.
Safety shift is what I think you are referring to. It will change aperture or shutter speed if it detects you will over or under expose with your current settings. For example, shooting wide open with an f/1.4 lens on a sunny day, if 1/8000 isn't fast enough to prevent overexposure, it will change aperture.