https://www.googlenestcommunity.com/t5/Streaming/Regarding-an-issue-with-Chromecast-2nd-gen-and-Chromecast-Audio/td-p/686992
Known issue being worked on.
Uh, factory reset is a natural troubleshooting step before I go to the
forums looking for answers... Hopefully the fix will work anyways when
its pushed out.