The preference doesn’t actually do what it says, as mentioned in the original post. Starting with macOS 10.12, it doesn’t always download all the attachments, and even ones that did download (because you viewed the message) may get purged later.
The only time I’ve seen the input switching not work is if there’s another app that keeps forcibly changing the input back to the AirPods. Some games and virtual environments do that. If you contact [email protected], I can look into what the issue might be on your Mac.