is it a bug oir a feature? In the meantime I found a workaround in the Joomla backend:
> select Content - Articles - Main Articles ("Hauptbeiträge", 3. item)
> for the startpage article select Options - Introducing Text ("Einleitungstext", 5. item)
> make sure that this option is set to "show / anzeigen", even if <ou don't use the "read more" feature.
With this setting the full text is visible, at least on my site.