I've done some troubleshooting, and noticed some weirdness. There are 2 use cases below:

common elements:

layout.phtml - has call to headStyle()->toString(2)

Now, if my View & partial look like the following:

test/view.phtml

<?php $this->headStyle()->appendStyle('#foo { color: purple; }') ?>

should be purple

test/_partial.phtml

<?php $this->headStyle()->appendStyle('#bar { color: green; }') ?>

this text should be green

Everything works fine... HOWEVER, if you remove the headStyle() call in view.phtml, you don't get any styles in the layout at all. Stranger yet, the current view object actually has the reference to #bar.