Light leak on a moveable object

Hello there

I have a simple static scene which works great with Directional (stationary) and Skylight (static) lighting.
When I place a basic moveable object (cube) and build a light using the Lightmass importance volume - I get light leak only around the area corners, if I place the cube in the centre of the area - all looks fine.

Does anybody know the solution how to avoid this light leak on the moveable object?

Many thanks!