Notifications arrive but messages are missing on iPhone (visible on Mac)

Messages appear on my Mac but can be missing from the iPhone’s Messages app. The iPhone sometimes displays a notification for the incoming message, occasionally showing the phone number instead of the saved contact name. Opening the notification takes me to Messages, but the message itself isn’t there.

Troubleshooting I have already tried:

• Restarted the iPhone.
• Turned iMessage off and back on.
• Signed out of both iMessage and FaceTime on all devices, then signed back into both services on all devices.
• Checked the phone number and email addresses in Send & Receive.
• Used Sync Now for Messages in iCloud.
• Turned Messages in iCloud off and back on.
• Disabled message filtering.
• Replaced the iPhone, set up the replacement as a new phone, and activated my existing phone number on it.
• Allowed time afterward and checked again.

I found a similar discussion where other users report that standard resets provide no lasting solution: https://discussions.apple.com/thread/256364427

The problem persists despite these steps. Has anyone found a lasting fix, or had Apple identify the cause? What specific diagnostic information helped Apple investigate this notification-without-message behavior? I’m looking for a next step beyond repeating the resets and sign-ins above.