Very interesting:
After more testing without mods and cleared thumbnailcache as always, I can confirm that this issue only exists for me in the new Businesses and Hobbies EP world! (Edit: I did not test all worlds, just picked some at random)
I can without issues enter the basements of other words, but not in the new one, the camera stops at ground level and doesn't let me go into the basement. I have tested in that preset lounge lot in that world, but also when using the gallery to add new builds i still have the same issue.
Not sure if that is connected too, but I also experience a lag when panning the camera around that new neighbourhood, as if some textures load too long (could be something else than textures) and impact the main thread. I'm on a beefy computer and have max graphics on, and my fps only goes down to 60-70 when it starts stuttering. Usually when my game lags it is because fps is down at 20, but this one is stuttering when panning the camera around even though the fps is still high enough for a fluent experience. Not sure if that is related, but again only seems to happen in the new world.
I'm using Sims 3 camera in build and in live mode. No mods, no changed camera ini and i have done a game repair before that too. I had the same basement issue on PC and Mac.