Shadow artifacting when I bake static directional light

When I bake a static directional light on my static mesh, I have this weird issue where one of the shadows gets all messed up, while the other shadows are fine. Pics:

The top picture is immediately after the bake, then a few seconds later, it turns into the second picture. The shadow that is messed up is cast from a window frame off to the side. In my scene, there is only the static directional light. The map is a plane with masked opacity alpha material and is set to have a 512 light map.

It’s really weird that only this shadow has the problem, but the shadows that come off the pens and headphone are fine. What causes this artifact and how can I fix this?