It was noticed earlier (maybe version 13) that if you place a custom material folder with the same name as a SU default folder into your user/materials folder, then SU will only show the default folder. I rename all my custom folders so then the material browser will show both.
My browser is full of cladding (the default) and cladding 1 (my cladding folder that I have been adding to for years), roofing and roofing 1 etc. It makes the browser crowded but it works.