Please, check if your presets are using particular material names. On the library loading process, materials are merged from the Max file, so if exist others in the scene with the same name may be overwritten.
In the Library Browser there is "On duplicate material" selector ("Use scene materials", "Overwrite", "Ask to user"), but it only defines the behaviour in case or conflict, don't solve it.
For our presets, we use unique names as: _fp_grass_a, _fp_daisy, etc. Also for the next Forest update, material presets can be stored in a .mat file, that eases the organization task.