I reviewed and updated the Finnish translation with Poedit.
As suggested, I deleted the files:
graphene-fi.mo
graphene-fi.po
in the folder wp-content/languages/themes/
and updated the files:
fi.mo
fi.po
in the folder wp-content/themes/graphene/languages/
WP did not, however, use the updated translations.
When I updated the files in the folder wp-content/languages/themes/
the updated translations started working again.
The translations here:
https://translate.wordpress.org/projects/wp-themes/graphene/fi/default
seem to be very incomplete.