Workaround update: selected_value works as intended, if you set it from code first. So if, in your init method, you do:
self.drop_down_1.selected_value="foo"
…then it all works (assuming "foo" is one of that component’s items). And from then on, selected_value will reflect the value selected in the DropDown.
(Obviously, this is just a workaround, and we’ll get the actual bug fixed soon! But I thought I’d tell you how to work around it in the meantime…)