Thanks for taking a look at it, the actual curve I need looks like this:
It’s one in a series of shape keys that make up an eyelid blink, and I only changed it into a straight line for simplicity’s sake, and to see if it was anything to do with it being a curve (which it seems not to be). I made a demo file a while back which doesn’t exhibit this problem, and I can’t seem to reduce the file any further so perhaps I’ll report it as a bug and see what happens…
(c: