Ideas
After my investigation, it turns out that "for rent" lots work like cloned (which makes sense), but they take up a lot of space for the game cache. What lots + a many of families cause the background to be set to default. Even adding a new plot that is not "for rent" can cause this problem (I posted a video on YT below) because the "cache" is full.
I even test that TOOLS maybe make problems - no it is't.
With or without mods results is the same - you have more (not default) lots on save (for rent and penthouses are heavier) and families - default backgroud will be appear.
I REMIND you that the NEW DLC introduces a NEW world (and a new type of lots), which can result in you having this error right after installing the new EP because the cache will fill up.
I had it after installing Lovestruck when release (L&D start on new save)
So EA, please fix that, add more "memory" - With each new add-on the game runs worse and worse.
If you have the courage to sell "Get to work 2.0" at least give us a chance to play it without any problems