Can't Delete Grid Fill

Can't tell if this is a bug or a "feature" I've never encountered before. Can't find a solution via Google.

I'm working on a map and this fill just appeared on one of the shapes - a red grid that can't be edited or deleted. It stays at the same screen resolution regardless of zoom level and doesn't respond to changing the shape's background fill, even to "no fill". Copying the shape into a new document does nothing. Opening the curve does nothing.
What is this, how did it get here, and how do I get rid of it?