Carlos, OK I've got a little more info.
Attached is a zipfile containing a MAX9_32 file with ForestPro objects and their splines. When you open the file, you will see the FP and splines properly displayed.
When I try to merge all objects in this file into a new file, the splines disappear. I can't make them display no matter what I do. They seem to still be there because the FP has the spline shape and you can select the splines via the layer dialog. But they do not show up in the Selection Floater or in the viewports.
If I get rid of the FP objects entirely and try to merge just the splines into a new MAX9_32 scene, they also come in as invisible.
Have you ever seen anything like this before? The splines seem to be corrupt somehow. Originally, the splines and the FP objects resided in one layer. This layer was set to 'Display as Box'. The splines and FP objects were set to 'By Layer'. Perhaps this corrupted the splines somehow?