3ds Max Modeling :: Hair And Fur Among MrProxies?

I'm trying to grow some hair and fur among proxy objects.I was forced to convert almost everything to mrproxies to be able to navigate and render it out.

When i was done with that my boss wanted the last finish on the scene, and that was to add a little coat of fur on the sea bottom. I then chose to open a new scene in max, and xref the main scene with all the mrproxies and then grow my. That way i saved allot of calculations and it speeds up my viewport. I also want to render my fur separate so the main scene just gonna act like a mask during rendertime.

The problem comes now. When I render the scene everything whent as expected, exept the masking. It looks like the mrProxies is "planes", How comes?

Is this just how proxies work or does it exist a workaround?

Edit: The hair and fur is "buffer" type.

3ds max 2012 - Mentalray

Edit 2: The issue only appears when using buffer. mrPrim and Geo maskes fine. So maybe its the way the hair is calculated and not the proxies.