So, IDK how true this really is but I recall reading that the optimal angle for light to hit a sensor is different than for light hitting film. So that's a factor in the image output.
it's kind of the reverse problem of LCD viewing angles. film had no structure, but each pixel on a sensor is at the bottom of a tiny little well looking out. the effect is exacerbated on mirrorless with a shorter flange distance.
the modern Nikon sensors are currently the best here, being designed with adaptation in mind. but it doesn't matter so much, the worst you might see is a fractional stop of additional vignetting. it matters even less if you're adapting an SLR lens with a deeper flange.
Never heard about angle difference but I'm no pro by any stretch. Maybe this is related to some nuances in optics for mirror vs mirrorless rather than film vs digital.
