Maya :: Group Node / Child Mesh Visibility

We recently upgraded from 2008 to 2012. Sometimes, on saving a scene file, some of the meshes within a group node "disappear". They basically behave like they're on a hidden layer -- even though they're not. You can see the transform and shape nodes in the Outliner, but in the viewport they're gone.

I can fix this by un-parenting the missing meshes, at which point they reappear -- and then when I reparent them as children of the original group node they're fine. But when I save the file and reopen it, the meshes are (sometimes) invisible again.

You might think the group node itself is hidden or attached to a non-visible Display layer, but it isn't. And there are other meshes within the group node that show up fine.

How I can permanently avoid it? I'm working on fairly complex environment scenes, and having to selectively unparent/reparent meshes each time I open my files is a real hassle.