For instance, here both slots, rendered in the shadow DOM, are visible on the page:
<slot name="inactive" hidden></slot>
http://jsfiddle.net/vu5qderm/ seems to work in STP67. Could you post a sample HTML where this problem is reproducing?
Closing this for now since I can't reproduce. Please feel free to comment the bug if there is a website or a sample HTML / page where this problem reproduces.