Hi Giacomo,
If you could make a staging site mirrored to your live site, we would glad to debug/troubleshoot this issue for you as we really want to help you on this
as we cannot replicate this issue locally, it might happened of any possibilities.
However the file that render the dropdown search result is actually in the file:
.../components/com_easysocial/themes/wireframe/search/mini/item.php
where mainly the $item was from:
.../components/com_easysocial/themes/wireframe/search/mini/default.php
and get from result:
.../components/com_finder/models/search.php
By the way, do you have try to re-index the smart search by any chance?