Jeremy, It seems to me that you're here not to help but to deny any possibility that the issue lies with Spotify app.
If you were trying to help, you would've at-least raised this with Spotify devs and tried to figure out where the issue is.
I've been able to reproduce this issue on Samsung Galaxy Note 20 ultra 5G, OnePlus 8 pro mobile devices and on Bose QuiteComfort earbuds, Sony wf-1000xm3, Sony wh-1000xm3 headsets. With any combination of these devices Google Assistant + YT music / Tidal works totally fine.
It's totally ok for you to say that you don't care about resolving the issue - we then have the option of determining whether to switch to another service or to endure the pain that is Spotify. But please don't make it seems like you're trying to help when all you're doing is blaming the issue on everything else.. Pretty bad outlook for a "top" music streaming service to have, IMO.
Jeremy, thanks for your replies, but we cannot contact Google Assistant because Google Assistant is not a manufacturer. Google Assistant is a system app on Android made by Google.
This is a Spotify bug and not an Android/Google/Google Assistant bug because this happens only with Spotify, not with other streaming apps like Youtube Music or Tidal. I've detailed steps to reproduce in the original post, but I'll repost them here so you can try them out and verify that this is indeed a Spotify bug.
Steps to reproduce:
1. Invoke Google Assistant. I do this by triple tapping on my earbuds (Jabra Elite 75t)
2. Ask Google Assistant to play a song on Spotify.
3. Try pausing the song through your bluetooth device (on my earbuds, I single tap; on my smartwatch, I click on the pause icon) - the song will not pause regardless of the bluetooth device used.
To verify that this is a Spotify bug, you can ask Google Assistant to play the song on Youtube Music. As you will see, after doing that, you can still use bluetooth controls to control the music (unlike Spotify).
I can confirm that Spotify is the issue. NOT Google assistant! I'm using a OnePlus 7T. YouTube music and Bluetooth controls work perfectly fine with Google assistant however Spotify is just a mess. Has been since November. The dev team really needs to up their game!
I have the same problem on my Galaxy Note 10+. If I start Spotify and manually select music my car controls work. After invoking Google assistant to play a song I have to reboot to get my car to control the music again. This ONLY happens with Spotify, not any other music service app. This use to work just fine. I'm the QA Manager for a well known media company and I can tell you this is a Spotify issue. Directing people to contact Google is irresponsible and honestly ridiculous. If Spotify has widespread issues with this which obviously is the case, any good customer centric company would work with Google to resolve the issue. Maybe we should just switch to Amazon Music or Youtube Music since their apps don't seem to have this issue.