facing same issue in my case as well, can anyone please help ?

Three things usually trigger this pattern (works in debug, silently drops in release):

1. Custom parameters vs custom events — different rules

• Custom events auto-register. On a brand-new project they appear in the dashboard within 24–48 h. You've waited past that, so this isn't the cause.
• Custom parameters (the dimensions/metrics you attach to events) must be explicitly registered under Firebase Console → Analytics → Custom Definitions. The event still fires with the parameter attached, but the dashboard drops the parameter column until it's registered. Most people miss this.
• User properties set via setUserProperty — same rule. Must be registered in Custom Definitions. Max 25 per project.

2. ProGuard / R8 stripping the measurement classes

With minifyEnabled true and no explicit keep rules, R8 can dead-code-eliminate the Analytics dispatcher worker. Your in-app analytics service logs "success" because the event enters the queue, but the dispatcher never runs — nothing reaches the wire.

Add to proguard-rules.pro:

-keep class com.google.firebase.analytics.** { *; }
-keep class com.google.android.gms.measurement.** { *; }
-keep class com.google.firebase.messaging.** { *; }
-keep class com.google.android.gms.common.internal.** { *; }
-keepattributes Signature
-keepattributes *Annotation*

3. Analytics collection disabled flag or consent mode

Under GDPR / consent mode, if analytics consent isn't granted, the SDK silently discards events in release builds. Debug mode bypasses the consent check, which is why you see them in DebugView but nowhere else.

Explicitly enable from Flutter:

await FirebaseAnalytics.instance.setAnalyticsCollectionEnabled(true);

And check AndroidManifest.xml — no library should have injected this meta-data (or it must be true):

<meta-data
    android:name="firebase_analytics_collection_enabled"
    android:value="false" />

Direct answers to your questions

Do custom event parameters need to be registered manually in Firebase Console → Custom Definitions before they appear in the dashboard, even in release builds?

Yes. Events auto-register. Parameters do not.

Is there a known delay longer than 48 hours for first-time custom events to appear in a brand new Firebase project?

Events: up to 24–48 h. Past that, the issue is parameter registration (not a delay). Parameters: never appear without registration.

Does minifyEnabled true with ProGuard affect Firebase Analytics event dispatching, even with -keep class com.google.firebase...?

Yes, if the keep rules don't cover the measurement service worker specifically. The event enters the queue but the dispatcher gets stripped. The rules above are the minimum safe set.

Debug order: first check whether release actually dispatches events via adb logcat -s FA FA-SVC. If you see events dispatching, the issue is Custom Definitions registration. If you see silence, it's ProGuard stripping the dispatcher.