We don't have enough info to really figure this out. I've had several soft-in-the-corners lenses over the years. Komura was one. A fast xenar was another. Wide angle lenses are tough to design and maintain critical focus across the field. To solve this, you will need to reproduce the setup, but photograph something flat, maybe a sheet of newspaper. Try it with and without the polarizer and let us know.

Since you said it didn't become dim or have light fall off, I don't think you were running out of image circle. Also, with flowers you may have had a little bellows extension, giving you a larger image circle.