a8x0p9b1ko0a Does this sound like your issue? Not the Onsen part, the more general idea:
If so, please use that master bug thread. You can also try Crinrict's suggestion of changing the lot type first (and making sure it sticks), and only then placing the build.
I tried a few things off there and nothing worked. In the end I fully uninstalled and let it reinstall (which took around 3-4 hours as my internet is not the fastest) that seems to have cured it as the lots appear to be behaving now - even with all my mods and custom content installed - well most of it as some of it appears to have disappeared but that's another issue.
Thank you for ay least offering me a way to test things and try and pinpoint the issue.
