Problem with layerMask and shadow

Hi all, and Happy New Year !!!!!

I come with an over problem.

I've got a ground, and some meshes on it (this is a strategy game). I've got 2 viewports, 1 for the game, and 1 for minmap to have a global view.

I would like to show or not some meshes on the minmap viewport (like skydome, building to replace it with sprite, etc etc ...), I read about the layermask, it is working fine until I put shadow on my scene .... If I hide meshes on minmap, shadow disapear in the over viewport ...

Here the playground exemple :

You just have to switch between line 36 and 37 with comment to saw what I mean.

http://www.babylonjs-playground.com/#1WELNV#1

Some good idea ?