Hello,
When I create a AGSModelSceneSymbol with a dae file and symbolSizeUnits is set to .dips, the size of the object changes when I rotate the object (heading).
I assume you're trying to keep always the same width of the object. But when the object isn't circular and I rotate the object it's normal that the width of the displayed object changes.
I attached a sample project of a symbol with a marker. When I animate the heading, we clearly see that the object size is changing.
Best regards,