Hi guys!
I found a way to save everything that are missing.
Don't go into play mode. Click on the missing house and choose build mode without entering the game with play mode.
Once you enter the build mode right away, you will see your house again. Once you're in build mode, "enter save to my library" on the top of the screen.
then save the game.
Your lot is back.
With me, it started recently (October 2018). Thanks for your suggestion, but in my case it also happens by going directly into the build mode. (I just exit wihout saving and can't play that house.) Although the home may have been saved in my library from the past, it wouldn't necessarily make sense to put drop it there, as it would also be without any "handiness upgrades" (such as kitchen, bath, computer, etc. upgrades, as well as 'rocket ship') and valuable hanging "masterpiece" paintings, etc. would be missing, too. It's very annoying and I hope it can be fixed.
