problem with a Static mesh render when built light

Hello UE community,

first message for me, I hope I am on the right place.

I am starter in unreal with a VFX background.

I have right now a problem with a mesh,

done in 3dsMAX
UV in UV layout
texture in substance painter
imported in fbx 2013

all looks goods in these software,
When I import to unreal, I had these alerts :
I checked no seam, exported in triangles.

BTW, when I clic on doc on the right it goes to broken web links (no docs anymore).

in unreal no material, it looks like this :

this mesh is in ONE piece !

with texture like in substance all is fine :

and the light unbuilt look quite ok !
but when I buit the light : the most ugly result !!, even in production level.

at the origin, the mesh was build with objects not connected together, I thought the importance of small meshes created the lightmap
textures result.
So I did a whole retopology unifying everything to be sure to not have adjacent face/elements conflicts.

so apparently, the concern is at the lightmap ( built in auto) /build light step.

I read the page about the light map (that pushed me to redo the mesh retopo.

but without result success.

can the community can push me a step foward please.

PS : this is a VR scene, everything works fine on VIVE until now

Thank you !