Hey, I tried building on the lot with a clean Sims 4 folder and unfortunately the same problem is still happening for me. I started by building a room on the lot and then if I remember right I had also deleted two walls that were inside of the house in order to open up a room and this is when the bug started happening (if I try to make any other changes to my build like building a room or placing down an object like a tree they just disappear). I would suggest you try replicating the actions I described in my first post in the "How can we find the bug ourselves?" part if you haven't tried this, because in the second post I made I was just describing what happens once the bug is occurring. I think once or twice after the first time I experienced this bug it took a while of building rooms, changing wall heights, placing stairs, placing platforms, etc. before the bug was triggered again, but most times the bug occurred almost immediately after I started building.
By the way, when using my original Sims 4 folder I tested building on a different lot in the same save file and I was able to build a whole retail store with no issues, so the problem seems to only occur on my Horse Ranch build for some reason. So this is kind of good news, but I still would really like to get this problem resolved if possible so that I will not have to rebuild a whole new house on the same 64x64 lot from scratch and just hope that the bug doesn't start happening again.