It sadly did not work for me but I tried $args['eventDisplay'] = 'custom'; and it works now 🙂
My final code:
function my_relationship_query( $args, $field, $post )
// add and define tribe events eventDisplay to 'all' since it's predifined only to future.
$args['eventDisplay'] = 'custom';
// acf/fields/relationship/result - filter for every field
add_filter('acf/fields/relationship/query', 'my_relationship_query', 10, 3);
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