It's a coordinate issue. The flat-shade re-import is a gradient with the center point and the focus point in weird places.
Also: The bug doesn't manifest if there's no stroke around the gradient. In the case of stroke v.s. no stroke, the difference between the resulting pdfs is only the /FlateDecode stream at the begining of the file, which I am having trouble digging into.
More research:
It's a coordinate issue. The flat-shade re-import is a gradient with the center point and the focus point in weird places.
Also: The bug doesn't manifest if there's no stroke around the gradient. In the case of stroke v.s. no stroke, the difference between the resulting pdfs is only the /FlateDecode stream at the begining of the file, which I am having trouble digging into.