To merge the XRefs in the scene, just get the list from the main Max menu: References->XRef Objects. Select all of them, and click "Merge In Scene" button.
Anyway, we will revise the Forest code, to be sure the XRef/Merge option is as expected.
Also, exceptionally that would happen if the importing process fails for some reason (as missing references or plugins in the external scene), because we merge the objects in two steps: first creating a XRef, and then calling internally to "Merge In Scene". Unfortunately, the Max XRef system not always reports the errors occurred in this step.