Can someone solve this for me please?

When assigning an object like a grid to be a shadow catcher somehow the casted shadow from a smaller object and casted shadows from the wall behind it don't melt into one as they should but the smaller object's shadow is darker than the shadow from the wall.
But when the shadow catcher is disabled and the grid is rendered normally, those two casted shadows melt into one, no difference as it should be... Is it a bug in shadow catcher system or I'm doing something wrong? Probably it's the latter.

Thanks

Jason