Ideas
Came across this post while finally looking for a reason I was coming across this in my own game. It's definitely a laptop mode thing, the stair railings were normal for me after I turned it off. I have photos from before and after turning laptop mode off, and only specific railings are affected. Also, the matching fences are completely normal whether you have laptop mode on or not.
Some of them are harder to see. The ones that I found to be glitched(?) were the Metal Art Deco Railing and the Art Deco Railing in Stone from High School Years, the Iron Tomarani Twist Rail from For Rent, The Stair Rail of Ruin from Life and Death, the Saharan Stars rail from Courtyard Oasis, the Woodland Knot Railing from Enchanted By Nature, and of course the Fae-Away Stair Rail from Realm of Magic.
These are the railings with Laptop Mode turned off. I do usually play with mods and CC, but I removed them all before testing. Turning off Laptop Mode was the only thing that fixed the appearance of the railings. I usually play with Laptop Mode turned on because I play on a laptop, and with it turned on my game loads noticeably faster.
I hope this is helpful, and I do apologize for the graphics quality. Like I said, I play on a laptop and some sacrifices have to be made in order for the game to be playable 😓