Flatten is not only a transform to a bitmap, but also a way to apply the changes to a layer and reset its bounding box. For instance, a rotated layer by 45degrees, once flatten lost the rotation value and it's frame takes the current absolute position and size.
An API command to flatten a Shape/ShapePath is missing.
Currently we should work with this suggestion: https://github.com/sketch-hq/SketchAPI/issues/139
Flatten is not only a transform to a bitmap, but also a way to apply the changes to a layer and reset its bounding box. For instance, a rotated layer by 45degrees, once flatten lost the rotation value and it's frame takes the current absolute position and size.