Are all 4 corners white like that?
I would have thought vignetting, but hard to tell with the white spot "coming out" of the corner.
What do the other corners look like?

As for the numbers on the filter, it sounds like the enlarger is using the entire filter, rather than just the center portion.
If so, I think you need to get another set of Ilford filters and when you cut it to size, REMOVE the numbers.