The image on the left is before processing, on the right is after - without the vignetting removed.
Granted this example is towards the sun, but it is visible on shots away from the sun as well (you can see it in the thumbnails at the bottom of the screenshot).
I played around with some DNG files that others uploaded and it was the same on those.
No biggie in any case - very simple to remove, it would just be ideal if there were a lens profile available, as per the Vision and Vision+
The vignetting appears to be light falloff due to the design of the lens, rather than light being blocked by the matte frame. It's quite common in wider lenses for view cameras - there are filters to correct for those. The solution here would be to change the formula of the Inspire 1 lens to cover a larger image area.
That's my take from a few years of large-format photography.
Here is one I made today, it is not perfect, but in waiting for ADOBE to come up with one I needed one. Remember that if using Photoshop instead of Lightroom you can choose if you want only parts of the correction. /BKE