Problem light effect on wall

Hello everybody,

I have a problem with walls in Unreal. When i build light, i have a white zone at the top of walsl. I created a simple cube with default lightmass and same problem. My uvs are flatten in 3dsmax, and the top is merged with the walls. I use hdri lightning, but idem with simple directionnary light (static). Dimension of walls : 13m by 9m, so tried 256,512 ans 1024 lighmass. My lightmass important node is correctly placed, and i tried to add a lightmass portal but same problem. Help me !