This took some digging. Under the hood, InDesign considers the last item in your example file a "polygon" not a "rectangle", thus in5 renders it as an <img>.
There are multiple ways a rectangle is not considered a rectangle by InDesign (drawing with the pen tool, starting with another shape and using Convert Shape to convert to a Rectangle, etc).
in5 does have a rectangular validation mechanism (for rectangles that have been manipulated or converted, but InDesign still calls them "rectangles"), but I'm hesitant to apply it in this case because it could have unintended consequences.
Hopefully that explanation helps you in setting up your files.