this has never happened to me before and i've had no errors the game loaded up perfectly fine

but...

i went to create a new lot for a sim temporarily until i made enough money to move into a house i downloaded, (which is still in the neighborhood along with two other sims...) but when i place 'empty lot' down into the neighborhood it shows then and i can enter the lot and build a house from scratch but once i SAVE and exit back into the neighborhood... the lot is GONE like i never even placed it there?? i've never had this happen and i've built several lots of my own plenty of times

all of my other sims and their houses are intact tho, i went into one house, saved and exited and it did not delete the lot

this only seems to happen when i attempt to create a new one and i even tried just lying down a blank lot and creating a new sim to test it and it loads up the sim and the blank lot but once i save and exit both the sim and the lot are gone? they dont show in the neighborhood brower of SimPE either but my other sims do

i've been googling EVERYWHERE to try and find some kind of solution for this but everything seems to be related to CC or downloading premade houses? which is not my problem

tl;dr: placing down an empty lot, building, and exiting into neighborhood seems to be deleting the lot

if this has happened to anyone or anyone knows what to do, thanks!

That is a bad problem, and I know it's hard to imagine how custom content could cause it, but unfortunately the game is so complex the strangest things can be down to a bad bit of code in an object you're not even using. Have you actually eliminated bad CC, bad caches, and your SavedSim file as sources? All you have to do is:
Back up your game (but I hope you already have a backup from before the vanishing of that sim!)
Delete your cache files
Temporarily remove your Downloads and Saved Sims folders (Just save them to your desktop or another drive; make sure you completely remove, not just disable them.)
Try the task again.

You can also determine whether the problem is in the neighborhood or in the game code by trying the same procedure in a different neighborhood.

Once you can say: "I've done all that and the problem persists," people may be able to give you other testable suggestions. I hope so.

I wonder if it's possible you've reached a limit on character files, and if the game handles it by simply not saving new lots and/or sims?

I don't know how likely it is that one could hit such a limit, but it seems like it might be possible if it's a very old 'hood in which many many sims have lived out their lives. Particularly if you have BV installed unpatched?