This started happening just after the iOS 14.1 or 14.2 update. I'm not 100% sure if its a Spotify or iOS issue.
When my phone connects to my car via Bluetooth, Spotify resumes as normal. Except, it plays back through the iPhone's speaker instead of the Bluetooth source. If I try to switch the audio manually, it won't switch. The only option is to relaunch Spotify. I just discovered that pressing pause/play from the car will sometimes switch the audio but not always.
Thanks for searching for the answer you need here.
We recommend that you run a clean reinstall of the Spotify app on your iPhone. This step can usually solve connectivity issues like the ones you're describing. You can find the necessary steps here. It's also a good idea to check if there are any firmware updates for your car's infotainment system. You can reach out to the manufacturer for instructions on how to get the latest version with all fixes.
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“Music is a language that doesn’t speak in particular words. It speaks in emotions, and if it’s in the bones, it’s in the bones.” - Keith Richards