Alright, thanks. Maybe the actual film negatives will give some more clues when you get those back.
Hm, that's odd indeed. Given how sharply delineated the circular anomaly is, I'd expect it to be fairly close to the film surface. Almost sounds like some part of the collapsing mechanism got unstuck and slides back and forth inside the camera body. Not sure if that's actually physically possible, though.
Is the actual image extremely unfocused on the affected images, but in proper focus on the ones that came out alright? If the affected ones are blurry as well, this suggests that at least part of the lens stack is moving about in an unintended fashion.
Is it possible to inspect the camera from the inside, and the collapsing mechanism in particular?