Render adding additional unwanted shapes / shadows.. please help!

press the match image button to rebuild your lighting before doing the rendor... if that still breaks then yes its prob a bug with Dimension or the Obj you are using for a model

you can post the Dn file if you want me to test it

yes that is what it is doing i.e, you gave it 3 objects and rendor preview is trying to show them

example 1 = rendor preview

example 2 = rendor

example 3 = what you are doing is hiding objects from rendor but they show against the model in preview... the preview will get this wrong every time because it doesn't have the power to work out the shadows

p.s, how good a preview image boils down to how much graphics power your system has; my Ryzen desktop makes great previews that are almost as good as the rendor, my Windows laptop tries but can't get the shadows right and I have seen many Mac systems do a dog's breakfast... once you finish the rendor they should all look the same