My displacement filter is simply broken. I made the simplest possible grid for a test subject, and then a displacement filter which is solid 50% grey. This should, as a displacement map, do absolutely nothing. But when I apply it, it produces the transformation shown below. It's moving the lines around, fading then, and it also adds horrible pixelation in the middle. This from a map that should have no effect at all! What's causing this and what can I do about it? Thanks.
Update: Here's a PNG of the displacement map, as requested (sorry it's so boring, but I did say it was a totally neutral map):