I think it’s stored in a particular save, so that you can load as many times as you like, you’ll get the citizens stuck in the ground every time. (See the start of the video attached below.)
If it helps, here are two saves where it happens, to me. In the former, all my citizens are stuck; in the latter, my carpenter and trapper are spared somehow (and they don’t seem altogether concerned about the lot of his fellow citizens). In the second save, also note Lorna on the ladder to the scaffolding of the house that’s being constructed.
Also my log files after the second save.
Pretty high priority bug, if you ask me. It’s broken my only two games so far within an hour of play (well, combined with the UI freeze bug that caused me to save and load in both cases).
Oh, I also got an error message like @Aron0621 mentioned/showed each time, which can also be seen in the video, but it was slightly different. Again, the first save is shown at the beginning; the second save at the end, as it is what made me end up stopping the recording.