Moving the light really far away seems to mitigate the problem, so I can work around it. Bizarrely, the bug comes back when the camera is rotated, the opposite of when the light is close, then rotating the camera fixes the issue.
The shadows are really sketchy/pixelly though, and I don’t know if that’s a result of moving the light far away, or whether it’s more of a backbuffer size or antialiasing issue.
How do you set the backbuffer size of minimized renderer in dx11 anyway?