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Since my TV received an Android update I had no end of issues. The TV can turn off suddenly while watching normal TV or watching via an app. The sound often does not work and I have to do a full OS restart. The TV goes a funny colour from time to time on the home screen.
The update was received 2-3 months ago. I have been waiting for another update to fix it, but it does not seem like one is coming.
Should I take the TV back to the shop? I have had it less than a year.
It really depends on spotify cast implementation. Some uses a generic android player (youtube itself run a generic rendered on tv rather than running the "local" app) some (as you seems to have discerned) use the local app renderer. In this case it could be the spotify app that have some problem when managed by the phone casting.
If you want to spend time to investigate you may try uninstalling the spotify app on the tv....
In case the app is installed and running on the TV (either foreground or background), Spotify on the mobile device uses Spotify Connect to communicate, being similar to Google Chromecast. I can still perfectly control Spotify from the mobile without having to touch the TV's remote.
If you force stop the Spotify app on the TV, Google Chromecast is detected and used.
This is with Android TV Oreo and and iPhone.