Undesired vignetting can't explain why

Hi,

I developed a roll of Tri-X recently and some of the exposures came out fine, while others, specifically some shots of a water canal, taken from above, shooting over a railing and pointing the camera down towards the water, came out with awful vignetting in the corners.

I am not using a lens hood and the camera is a Bessa R3a with a 40mm lens.

Other shots in the same roll came out fine.

There's a slight possibility my camera strap was causing it, by hanging low, but if it wasn't the strap, what could it be?

My first guess was gonna be the strap

I did this recently but the vignetting isn't uniform around the frame.