Terrain paint disappears after leaving lot

This has been going on for ages, I don't understand what more info you could possibly need. Just had half of my train paint disappear after leaving the lot to playtest and then reloading the save. It was there when I saved before playtesting. It was there while playtesting. It was gone after reloading. Only part of it was gone. The first lot of pebbles and pavers remained, but the dirt I put down at the same time disappeared, and the pebbles I put down later disappeared. I had the same issue over a year ago in a different lot. Thankfully this one didn't have as meticulous shaping but it's still extremely frustrating.

Oh, and I've never had mods or CC, and it's a different save to the previous one with the same issue, in fact it's a very recent save (within the last few weeks) that I've been using purely for building until they fix the pet issue.