I have a closed in room that is supposed to have no sunlight in it because there are no windows to the outside.

However one side of the room is randomly lit up for no reason by the sun when it shouldn't be.

I followed this tutorial Link and it fixed the random lighting at the edges but this is happening now.

(The light in the room has nothing to do with it since it's the exact same with it removed.)

Screenshot:

I have this issue as well, been looking for a solution!

I've heard that reflective probes will stop this from happening, but still not had the time to check

I have this issue as well, been looking for a solution!

I've heard that reflective probes will stop this from happening, but still not had the time to check

I've tried using reflection probes but that just seems to make the entire room bright as day when I have just 1 light, not what I want.

I've heard that no one really knows a solution yet!

Guess we will see in coming days

use the brushmodel, includes your room, applies CSG Hollow (Selection-> CSG-> CSG Hollow).  Then applies texture  "caulk_shadow_primary". Rebuild the sun.  try, it work fine for me