Problem with Full-Color fill

I'm trying to assign this seamless vector pattern as a full-color fill.

It works, but then I want to adjust the fill using the interactive fill tool. As soon as I twist the pattern even just slightly, the outlines no longer look smooth, but rather like a pixalated bitmap.
It doesn't happen with other patterns.
These are very simply graphics, so I have no idea why it happens.

I'd appreciate any ideas on how to fix it!