Terrain paint disappears after leaving lot

Same issue here. Fresh save, all packs. Even a clean install of Windows. I build on a lot, save and go build on another lot, come back from said lot, and notice the paint’s gone from map view. I head back in and yep: the paint’s gone.

This is frustrating enough that I just went back to TS2 and 3. At least I know those games won’t erase my hard work.