Hi Jack,
The setting I am referring to is shown below. When I have it set to 1 my flood area will have the mesh patter shown to the right of it, and will show in the Gerber file the same way.
Now when I change the line width to 25 the flood is solid, and looks correct in the Gerber.
I talked to a support guy about this a few years ago and was told that it was in relation to line width being small to something else but I don’t remember what that was. It wouldn’t be a big deal if it didn’t mess with Gerber data but since it dose I keep an eye out for it. This is really all i know about it. I don't know why this happens.