Automated Preview Rendering of Materials only works when used inside a "Preset" library (*.lib4d file)
I personally prefer to have my contentbrowser library in "catalogs" (*.cat4d file) because they don't generate duplicates within a lib4d file and produce a hell lot of files.
lib4d files are a kind of container that can have c4d objects, materials etc in it, but it always creates a copy of the file added within the lib4d file
cat4d files are just catalogs that point to a location on your local drive, it can point to c4d files, jpgs, or any other file of choice, but it's always just a link to that file not a copy
when using catalogs it would point to an empty c4d file with only containing the chosen material and cinema can't know the difference to an ordinary file which would be perfectly fine showing the viewport as preview
If using the second method for materials, you have to add a preview thumbnails manually.