I am not sure I believe that... It is defo an issue present in the new app... The galaxy watch app and software was not updated... The only thing that changed was the Spotify main app. I will email them too.
I got a Samsung Galaxy Watch update around Christmas and a Spotify watch app update around then.
Although granted this broke before then.
Yes it's the Android app that changed but that's not explicatally what this means though. It's Samsung's archecture, they own Tizen. It's stands to reason Spotify will need to work with them.
Because annoyingly Mobile apps don't have released Change Logs like Desktop apps do we don't know what's different between 8.4.80.x (and newer) and 8.4.79.x. It could be Spotify expected Samsung to implement something to work with this new version and they haven't.
Take the responce as a positive, they're working on a fix.
There is actually an easy fix. Normally a Samsung Music widget lets you control Spotify in remote mode.
If you want to use Spotify widget/app instead open Samsung Music app on your watch, then click three dots and the phone icon. This will show you an available music sources with Spotify selected. Choose something different and then Spotify app and widget start working in remote mode :)