Is this a some sort of lens-camera alignment issue?
I have a A7R III and have been using it with both a Viltrox Lab 35mm f1.2 and a Viltrox Lab 135mm f1.8. This is the first time I see this kind of vignetting, the bokeh on some other photos shot at a wider aperture also seem a bit off. I took those pictures during a hike, I switched lenses a couple of times but never noticed anything out of the ordinary while shooting. I have not been able to recreate the issue ever since and other photos with the same settings and framing seem ok.
Edit 1: Most of the affected images are portraits, with some of the 135mm shots having the whole left side clearly darker than the rest of the image.
Edit 2: It seems to be a know issue with the EFCS, here is a link to a forum with a guy who seems to be having the same issue as me: https://www.dpreview.com/forums/threads/a7riv-linear-vignette-issue.4523565/