Hello there!
I found one solution to this, it’s a bit late I guess. May be it will help someone with the same problem.
Set in light source settings “Contact shadow length” to 0.1 (There could be some bugs with shadows, so you can play with its value. For me common values are 0.007, 0.05, 0.1). Also the wall should have the “Contact shadow” checked (it is by default if I’m not wrong).
It helped to get rid of the light leaking even through the planes for me!
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