Hi, been using SC for a while now, usually casting it from my phone or chrome browser to living room.
Have the following issue for as long as casting has been supported natively in app on multiple phones and multiple cast devices all running latest versions - basically the song will start playing from phone speakers and from the cast device simultaneously.
Pausing from the phone will pause the chrome cast but not phone speakers, usually stopping the cast then re-connecting will fix it, but still really frustrating.
Haven't seen a pattern in which songs trigger it, but seems to happens once an hour or so, usually when going to next track.
Currently using S9+, but also seen issue on S7, and Xperia Z3. Issue present on Chromecast 1, 2 and Ultra. I've also seen it happen on a laptop casting from chrome browser, so I'm guessing it's something in the implantation rather than device specific.
Also surprised there is no other mention of this issue anywhere on the internet that I can find in 8 minutes of googling, given how prolific it's been in my experience.