I’m sorry to bother you on this, but your plugin is definitely one of the best out there and it has so far suited many specific needs of our website like no other.
I deactivated the plugin myself, then added a standard WordPress search bar on the header. I kindly invite you to test it just another time. Let’s say, search for “tecnica” – results are here:
https://www.musicandbooks.edizionicurci.it/ram10/?s=tecnica
And filters show.
I’m clueless…