Setup: Forerunner 570 (42mm), iPhone 16 Pro, iOS 18.7.8.
Symptom: WhatsApp notifications arrived on the iPhone, and every other app’s notifications reached the watch, but WhatsApp’s never did.
Already checked, with no effect: iOS notification settings for WhatsApp (Lock Screen, Notification Center, Banners, Sounds), Scheduled Summary off, no Focus mode, “Share System Notifications” on in Bluetooth settings, reset WhatsApp notification settings. Garmin Connect has no per-app list.
Fix: the per-app setting is on the watch itself: Settings → Connectivity → Phone → Notifications. WhatsApp was disabled there. After enabling it, notifications came through right away.
Hope this saves someone a few hours!