The actual bug is in advanced-custom-fields/core/fields/relationship.php in line 496:
At the moment it is: $post_type_object = get_post_type_object( get_post_type( ) );
And it should actually be: $post_type_object = get_post_type_object( get_post_type( $p->ID ) );
Otherwise it is getting the post type of the current post, intead of the selected post for the relationship block…
Ok, it’s good to know!
Now I’d expect a comment from the creator confirming this is a bug and telling whether it may be fixed in next release…
Unfortunately I didn’t so far.
Actually I’ve not worked on that particular issue since I wroted the question, but I guess if I don’t receive any better suggestion, I’ll try to use the filter acf/fields/relationship/result to customize the HTML correctly. I’d try to grab the post type of each row and manually set the correct text for each row.
I think that should work. I’ll comment when I try…