@mishraka it is a mystery to me!
I can’t find any strange settings in your file.
When you don’t have your floor as Shadow Catcher (only Ceiling and Walls) it behaves as expected:

But as soon as you also make the Floor a Shadow Catcher, the effect is as you already showed.
There is not actually light coming through the Walls and Ceiling, otherwise the Monkey and Painting would be much brighter.
I’ve tried a few different Blender versions, but that didn’t help.
Maybe it’s a bug?
Does @jlampel know what is happening here?