Okay, really strange. I must say this was not really an idea with the obj, I just wanted to be sure that the rocks are not texture painted terrain or so.
But again, this approach with renaming the rock and all of its child objects (if any) to "Spline xxblabla" so the sim somehow "handles" it as a PP spline (?) did you try?
Maybe I'm on the wrong path but just if it makes a difference...
The rocks are named Spline XXX. Or... the mesh created when doing the mesh combine. I am really thinking this is a Unity issue, but I am not all that well versed in these things...