From what I can see, the Finder plugin is registering posts using:
index.php?option=com_gridbox&view=page&id=XX
Because there is no Itemid attached, Joomla’s router cannot associate the post with the Blog menu item (view=blog), so it falls back to the site root and produces the incorrect SEF URL.
Could you please update the Gridbox Finder plugin so it generates SEF URLs using the Blog menu item as context? For example: