What Am I Missing Here?

Recently I went out to try and photograph the Perseids. I had with me my Canon EOS R Mirrorless. BTW----This was my 1st attempt at meteor photography. For my lenses I had my Tokina 11 - 16mm as well as I had my Tamron 24-70mm. I was planning to use the Tokina Wide Angle because it is wider. As I was aiming at the sky and set the lens to its widest (11) I noticed a major vignetting thru the lens. But when I brought the lens into "16" the vignetting disappeared. Below are examples of what's happening. I wound up using the Tamron for the meteors and got a few good shots. But I'm perplexed about why I cannot use the Tokina at its widest. Is this lens not compatible with the EOS R or is there something else I'm missing? Thank You.

The Tokina 11-16 is designed for crop sensors, and it is only usable at 15-16mm on full frame cameras. This vignetting is expected at 11mm. I had the same issue when I got my 6D, and so I got the Samyang 14mm instead.