Same material, different appearance? + light bleeding

Okay! So, I’ve tried to align my mesh/verts to grid lines in Blender and…no luck, it didn’t work. Still had the exact same light bleeding happening where the roof met the wall.

Something interesting happened though. I noticed my hard drive had filled itself up to the brim and it completely ran out of space somehow (which is unlikely since I only a few programs installed). I tracked down the space hogger and it was the DerivedDataCache folder…which somehow grew to a whopping 39.1 GB. So I removed the folder, did a restart and went back into my scene and rebuilt it.

…and the light bleeding disappeared?!

Granted I still have some shadow blotchiness, but I can fix that. Why did removing the DerivedDataCache folder fix the light bleeding?