Some faces are darker for no reason

[victorbnl]

Here is how my object is in Cycles:

And after building the lightmap:

As you can see the face above gets darker as if it was under a shadow, but it’s not, and that happens even if the light is right above the face.

EDIT: the issue is even more noticeable with a higher resolution shadow map:

Project: https://www.mediafire.com/file/fwo3ixuoj2lgn14/bedroom.blend/file