No media controls appear in the notification area during playback for /e/ (which uses microG to replace Google’s proprietary libraries) users.
The playback controls show for Iceraven, a Firefox fork

/e/ OS’s default web browser

We would expect to have similar behavior as when I’m playing music through a web browser or other music apps.
React Native WebView uses the phone's default browser.
The stream repository uses MediaSession to handle playback controls in the notification center, which works as intended on Android and iOS mobile devices.
We have inquired on the /e/ forums about this issue.