Which MDL do you mean - the tree or the background object? If that one contains an external transparent texture, the reason for the problem is clear.

Otherwise it could indeed be a bug. But the external texture alone can not be a reason for the problem. You can send the project to the support - please not only the model but a level where the problem is visible. I'll be away the next 10 days, but I'll look into it after my return. The address is support1(at)3dgamestudio(dot)com.