I have never come across this issue either. The first thing that comes to mind is it may be some kind of custom content/mod interference or something.
Please forgive any repeated troubleshooting steps.
Just curious, have you tried:
1. Saving the house to your library, and try loading the house in a new test game. - No need to save the test-save game. Does it fix the issue?
2. Try taking out your mods folder, clearing the game cache and try loading the house in a test-save.
3. If not it might be something is corrupt in your Sims 3 folder. Rename your current folder to Sims3Old. Open and close the game so it creates a new Sims 3 folder.
The goal of this test is to get rid of the plus signs in your house.
4. Copy your Library folder, and your option.ini file into your new Sims 3 folder.
-- Please note your custom content and store content will not be there. -- It's okay, this is just a test.
5. Start a new test-save. Load the house from the library. How does the house look? Does it look better?
6. If so, you might want to troubleshoot the files in your mods folder by going through what custom content and mods you have.
When I've had issues with bad CC and mods, I've had to go through them in small batches, clear my game cache in-between each batch until I was finished finding what bad cc or mods I had.
** Searched on the issue and found the reference links below.
It may be a good idea to keep with your new Sims 3 folder, and copy in the folders and files described in the post below. As this will help get rid of all traces of outdated mod.
What Files/Folders to Add Back In
https://www.carls-sims-4-guide.com/forum/index.php?topic=20732.msg401814#post_FF2ABTry a test save, go into build/buy mode to place a few things to test if you get the plus signs. - I'm hoping not, but if you do it might be something to do with that specific house file. It's been a very long time since I've heard of a house file being corrupt, but it does happen from time to time.
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Reference Links:
Game Help:TS3 Finding Problem Custom Content
http://simswiki.info/wiki.php?title=Game_Help:TS3_Finding_Problem_Custom_ContentCustom Content - Blue + Sign Every time I put something in Build/Buy Mode??
http://modthesims.info/t/497476