IOS Saves Credentials for Deleted Apps

If I delete an app from my phone and then reinstall it the app will log me in with my previous credentials automatically, meaning the app information was never fully deleted from my phone. That behavior seems to contradict what iOS says when you delete an app - "Deleting "[app]" will also delete all of its documents and data. This action can't be undone."

I deleted this app literally years ago and yet installing it today I see that after all this time it still logged me into my old account that was connected to the app way back then.

Regardless of how the app is designed, why is iPhone saving data for an app that was deleted, even after multiple restarts and multiple years later?

Worth mentioning that I do not use iCloud whatsoever and nothing should be synced anywhere. Also, this credential is NOT stored in the password vault/manager.