「ソフトフェイル」状態のセルフホスト型TWA - フルスクリーンUIは動作するが、ネイティブ委任が失敗する

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I’m trying to get native notifications working for a TWA of my self-hosted Discourse site and have run into a very strange “soft fail” with the Digital Asset Link verification.

Here are the exact symptoms:

  1. The app launches and runs full-screen without the URL bar, which suggests the asset link is partially working.

  2. However, a “Running in Chrome” toast message appears on every launch.

  3. Most importantly, when the PWA requests notification permission, it shows the browser-style prompt, not the native Android dialog. This proves that native API delegation is failing.

This behavior is consistent on all test devices (Android 12 & 13) and with APKs generated by both Bubblewrap and Microsoft’s PWABuilder.

After extensive debugging, I have confirmed that my entire client-side and public-facing server configuration appears to be perfect. The issue seems to be a subtle server-side problem that only affects the Android validator.

Here is what I have already verified:

  • Signing Key & assetlinks.json: My android.keystore SHA-256 fingerprint is a 100% exact match with the fingerprint in my live /.well-known/assetlinks.json file.

  • Server Response: My server serves the assetlinks.json URL with a 200 OK status, the correct application/json Content-Type, and no blocking CORS headers when checked with standard web tools.

  • Android App Config: The generated AndroidManifest.xml is correct and contains the standard com.google.androidbrowserhelper.trusted.DelegationService.

  • Discourse Admin Settings: My admin security settings for cors origins and Allowed crawler user agents are both empty.

Given this specific “soft fail” behavior, my question is:

Is there a known Nginx rule, firewall setting, or a subtle server response issue (like a slight delay or a non-standard header) in the default Discourse self-hosted setup that would cause the Android validator to grant a “partial trust” (allowing the full-screen UI) but deny the higher level of trust required for native API delegation?

I am at the end of what can be diagnosed from the client side. Any insight into the server’s deep configuration would be greatly appreciated.