Get the world coordinates of get_used_rect() for an isometric tilemap?

Question

I'm trying to get the full rect of the used part of a tilemap and store the four sides in an array:

edgetiles = [
    map_to_world(get_used_rect().size).y, \
    map_to_world(get_used_rect().position).x, \
    map_to_world(get_used_rect().position).y, \
    map_to_world(get_used_rect().size).x
 ]

The problem is this gives me some strange values that don't correspond to the world coordinates of the isometric tilemap rect at all. My tilemap settings are:

Does this require some special setting to grab the edges in world space?