Hey @Magwal, welcome to the Community.
Hope you're doing great!
It might be that your Spotify credentials are saved in the iCloud account. Keychain specifically, and iOS can sure work pretty seamless across different devices, it's one if it's strengths.
Spotify's are also working on making it easier for users to log in. For example the magic email link you can receive so you can login without a password it's one example.
It could also be something else though, it has happened to me a few times, it's just to make the user experience better.
Hope it helps 🙂
Help others find this answer and click "Accept as Solution".
If you appreciate my answer, maybe give me a Like.
Note: I'm not a Spotify employee.