In this case, featured images for albums which only contain other albums need to be moved to a sub-directory to avoid being displayed.
Folders with no images
Albums with no images are hidden by default. This is a design decision to keep the structure as simple as possible and hides pages like about.md or imprint.md from the album list without the need of defining a layout/section for each gallery.
@baekgaard made a Pull request
about handling of empty albums, which allows to modify this behaviour (which, for simplicity reasons, is not merged).