Did you make sure that the notepad file in that folder is indeed a "Material" file and that it is named "preset_8" (or other number)?
Sometimes when you fiddle with the coding, the text editing program will save the file as a .txt file rather than a .material file.
If you checked and the file type reads" MATERIAL File, then please post your code here in case there is a small typo that's gone unnoticed. I know that sometimes a missing bracket or something like that can cause this or a marshmallow preset.
Also, make sure your folder is called "preset_8" as that is also a way of getting them to poof.
The case of a preset number disappearing has to do with folder and material file naming rather than image naming. If your images are wrong, then the preset will be all white, but should still be there to select, so it sounds like a case of naming the material file, its extension or folder name.