I think I have found something. The theme uses the same filter to override the search form. It's probably executed later then the plugin filter, so it's simply overwritten.
Can you try disabling line 341, which should be:
add_filter( 'get_search_form', 'road_search_form' );
to disable, change that to:
//add_filter( 'get_search_form', 'road_search_form' );
or you can remove it, if you prefer that. Let's hope this changes something.Best,
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