Android App: No notifications and syncs only when opened (despite 15-min auto-sync enabled)

I'm having trouble with the Thunderbird app on Android. I never receive any email notifications. It seems that the app only syncs when I manually open it, even though I have the auto-sync interval set to every 15 minutes.

I've already checked that notifications are enabled in my system settings, and battery optimization is disabled for the app, but the issue persists.

Is anyone else experiencing this, or does anyone know how to fix it? Thanks!