I have two lenses that I use with 8x10. One is a 300 mm f/8.5 Fujinon-C. The other is a 159 mm Wollensak f/9.5. I believe the 159 just barely covers 8x10 so the image circle may be the case with it. Image circle for the 300 is 380 according to the Badger Graphic site.
I am seeing this with both lenses. I checked and it does seem to be the corners and along the egdes of the film.
The camera is an Agfa/Ansco. I should have said that I am going to support the bed of the camera with the long lens support. No, I'm not shooting portraits, mostly landscapes.