@DeanXeL Wow! Thank you! You put quite a bit of work into that.
I tried both potential solutions you gave me. They do remove the phantom shadow. However, using just one or the other without further modification really doesn't work. It either renders the second floor unusable or makes it impossible to have even a semblance of an enclosed room. I'll also need to fidget with windows and such. I might have to simply lower every platform downstairs and remove the grand stairs I created out of platforms on the outside. I'm not sure yet and don't have much time to test this weekend, but I'll keep these in mind for whenever I manage to get back to this build. Or just hope the team somehow fixes this bug.
Thanks again for the help! I uploaded the lot in case you're curious :
https://ea.com/games/the-sims/the-sims-4/pc/gallery/1F17B2A81E0611EC88DD7DB6A3FFB678?category=all&searchtype=ea_origin_id&sortby=newest&time=all&searchquery=SerraNolwen&max=50&maxis=false