All right. Figured it out.
Anyone needs to solve this: “all” doesn’t work (anymore?) but “custom” does the job and shows all events (upcoming and past).
Here’s the updated 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'] = 'all';
// acf/fields/relationship/result - filter for every field
add_filter('acf/fields/relationship/query', 'my_relationship_query', 10, 3);
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