Hello,
Doesn't sound like you are doing anything wrong, sounds like a bug. Can you please provide more details about your working environment? 3ds max version, Ox version and build and OS.
Are you able to reproduce this is a clean scene? If not, please provide a simplified version of the problematic scene, just enough to show the issue. You can send me the scene to jeordanis.f (at) ephere . com, with a link to this post.
Jeordanis Figuereo (Product Designer. EPHERE Inc.)