Fix FCM Background Notification Bug

My Java/Kotlin Android app receives Firebase Cloud Messaging alerts perfectly while it’s on-screen, yet they arrive inconsistently—or not at all—once the app is backgrounded or killed. I trigger every message straight from the Firebase Console, so the payload is just the standard notification block with no custom data. What I need from you: • Track down the exact reason notifications vanish on newer devices (the issue is most noticeable on Android 13+). • Correct any mistakes in runtime permission handling, notification channels, priority, or power-management settings so alerts land 100 % of the time whether the app is foreground, background, or closed. • Supply a concise summary of the root cause and the fix so I can avoid the pitfall in future builds. The codebase is clean, the problem is isolated to FCM, and an engineer who knows their way around Firebase, WorkManager, and modern notification APIs should wrap this up in a few hours. When you send your proposal, begin with “FCM” and in one or two sentences tell me the very first thing you would inspect to diagnose this.