iOS Critical Notifications # iPhone 12 (15.4.1) does not play custom notification sound

Question

1. Our app equipped with critical notifications support and play custom sound (siren) for critical events - https://medium.com/@shashidharyamsani/implementing-ios-critical-alerts-7d82b4bb5026
2. Everything works well if user enable both "Allow notifications", "Critical Alerts" toggles in iPhone settings.
3. iPhone receives critical notifications
4. But the custom sound not played for critical notifications, if user enable only "Critical Alerts" as shown in below screenshot

Refer attached image for - iPhone Settings

Not sure whether this is expected behavior or an issue.

Please let us know if anyone supporting the critical alerts feature and encounter the same issue for custom notification sounds.

Thanks in advance...