I have the same problem
Moreover, if you open a folder through the file menu, then thumbnails are created. If you open the library after that, they are not there.
I deleted the bases and sketches, updated the library and noticed that the sketches are downloaded from the Internet only when you select a series.
If you disable the Internet after updating the library, the thumbnails will not be loaded and will not be created
I decided to delete it all by installing the version of LibreELEC-RPi2.arm-8.2.5.img.gz, and recreating the library
I hope that fix