Why does this particular lens produce sunstars with long spokes?

This image was shot with a Sony E 20mm f/2.8 on a Sony a7. The lens is intended for an APS-C sensor, but I removed a baffle to shoot on the full-frame body.

I understand the basic physics of sunstars. My question is why the sunstars from this particular lens have the giant spokes. This is not due to anything about the composition of this particular shot; the lens always produces sunstars like this.

Edit 1: Yes, the sensor was filthy. Other lenses used around the same time had normal sunstars. I’ve since cleaned the sensor, and this effect still appears with this lens.

Edit 2: As requested, images of the font of the lens with the aperture at f/16, the smallest available. The first image has a UV filter in place; the second does not.