Hi, actually there are not known issues with such behaviour.
What is the state of the Forest objects when that happens ? Can they be selected and edited, dissapear completely or what ?
Can you see the trees on the viewport ?
Are you using library models or your own objects ?
Forest stores their trees in a hidden layer (forest_templates), as meshes or XRefs (depending of the option selected in the Library browser). Perhaps some Max operation is removing them from the scene.