As @tyczj commented to use "health", this was it. On Android 13/14 you need to run Activity Recognition in a Health FGS. Two tweaks fixed it:

<uses-permission android:name="android.permission.FOREGROUND_SERVICE_HEALTH" />

<service
    android:name=".BackgroundDetectedActivitiesService"
    android:exported="false"
    android:foregroundServiceType="health" />

Also make sure:

• ACTIVITY_RECOGNITION is granted (runtime on API 29+)
• POST_NOTIFICATIONS is granted (API 33+)
• Use ActivityRecognition.getClient(context) and a FLAG_MUTABLE PendingIntent

After that, updates started flowing.