I run into the same problem when I print negatives exposed with ultra-wide lenses. It's called light-fall-off. It's also easy to correct for when printing the negative by burning in the corners of the print during the exposure. Cut a small disk out of some card stock about 2 or 3 inches in diameter, attach a length of stiff wire to it, then block the exposure in the center part of the projected image and give the corners some extra exposure (10-20%). Move the disk slowly up and down in the light path and allow more exposure in the corners. Do this as a second step after you've made your initial exposure. When you find the correct time ratio for the two exposures your prints will be exposed correctly from corner-to-corner. Ansel Adams often gave his prints a little extra exposure around the edges.

You can keep working and save paper by doing this while you investigate the issue.