This is not a new topic/issue, I know that, but after searching the forum and the rest of the internet, I haven't found a solution or even an answer/reason why it happens.

Here's what happens:

After exporting the world and playing properly in the game, the empty high rise shells I've used on CAW to fill up my city, disappear as the camera gets close to them, just as it does on CAW, leaving only the base outline, and I have even painted those areas as Sims Non-routable.
Again, these are empty shells that I use just to decorate, in which I can't and don't plan building anything.

Does anybody know how to fix this?

Don't use shells?

Seriously. I have not experimented with this, but there is a switch in the file which designates something as a shell. I think it is the nature of shells to vanish as you get close to them. So if you mod the building file so as to turn the shell switch off, it might no longer work as a shell.

I eventually am looking at doing the exact opposite: turning solid buildings into shells when the camera gets real close in.

You'd probably need to mod the shells themselves to stop them fading. But there is also a cheat which may help temporarily:
fadeObjects [on/off]
http://simswiki.info/wiki.php?title...p:Sims_3_Cheats