September 2020
September 2020 - last edited September 2020
Okay, so I tested those scenarios I thought of yesterday.
1) Using the "Place Fence Enclosure" tool doesn't trigger the bug. Basement remained open both inside and outside the fence.
2) Using the "Place Half Wall Enclosure" tool does trigger the bug. Grass remained inside the the half wall square.
3) Creating a false "room" by drawing individual walls elsewhere and then picking them up one by one and placing them next to each other does not trigger the bug! We have a workaround! Still a massive nuisance, and the game not recognising it as a "room" probably creates other gameplay problems, but it at least looks the way it should!
Anyone want to test this and confirm my findings?
October 2020
Okay, it's been nearly a week and nobody has provided evidence that my workaround doesn't work for them, so I'm gonna go ahead and tag @crinrict that we have a workaround! Still clearly a bug that needs to be fixed, but you can get around it by using trickery to place the walls.
October 2020
I can see your issue with pictures and everything. I managed to make a video and upload it on youtube to show people. Here is my video to show you my problem!
March 2021
Bumping because this is still a thing and it doesn't seem to be on the laundry list or anything similar.