@IrisuChan Awesome job of testing and getting to the bottom of it! I would keep a backup, on your computer or in better yet a flash/external drive, of the the save that works for you now, to roll back to if the issue should re-occur. If the issue rises again you may want to checkout the workaround and chat at this bug report: https://answers.ea.com/t5/Bug-Reports/WORKAROUND-Lot-in-World-corrupts-Map-doesn-t-load-anymore/m-p/13299392#M263688
If it sounds like what happened in your game, originally then while testing it, you could add an 'XP' and click "Me Too" on the report. Thanks for your reply and hard work, and I'm so glad you were able to get back in your game, and in the lot you want to use. 🙌