Hi, so this is a little complicated but follow me...
If I play a group of songs without casting, or use an insecure connection and simply cast the entire tab that the web app is running on... the play queue works fine.
If I play on a secure connection (ie. through my secure reverse proxy URL) it seems to support casting differently... and the play queue does this:
https://i.imgur.com/8cpdLyT.gif
Notice how Lost in Echoes never moves yet parts of the Play Queue shuffle around.