Can't exit build mode on lot + game won't close

Alright, this feels like it's gonna be tricky to explain, but I'm going to do my best.

I have been building a passion project build from scratch on a separate save from all of my other games for a few months now. However, as of late, I cannot get a single thing in build mode to work properly. It started with me trying to add a basement; I would place down a square and then try to lower it by a couple levels, but it wouldn't even register that I was clicking. Then, in an attempt to force exit, my game got stuck, and my only possible way out was to force restart my computer.

I tried playing on an entirely new game folder, and build mode worked fine. I thought it was something with my mods, so I tried 50/50ing them and that had no luck. I loaded in another save in that same game and tried building, and it worked fine there as well. Therefore, it is this one particular lot on this one particular game, and I don't know what else I can do to fix it. Please give any assistance you may have. Thank you!

Note: please forgive the sped up videos, but I figured this was the best way to display my problem to others. The first is the broken save, the second is a different save in the same game.