OK Google search actions causes isVoiceInteraction() to always return false

Question

In the Google CodeLabs example for the Voice Interaction API, an activity is defined with the following intent filter (see step 6):

<intent-filter>
    <action android:name="android.media.action.STILL_IMAGE_CAMERA" />
    <category android:name="android.intent.category.DEFAULT" />
    <category android:name="android.intent.category.VOICE" />
</intent-filter>

When using the "OK Google, take a selfie" voice command, the intent is fired with the android.intent.category.VOICE category. This is shown in LogCat as:

02-26 15:32:42.423 779-6923/? I/ActivityManager: START u0 {act=android.media.action.STILL_IMAGE_CAMERA cat=[android.intent.category.VOICE] flg=0x18000000 pkg=com.example.android.voicecamera cmp=com.example.android.voicecamera/.TakePictureActivity (has extras)} from uid 10027 on display 0

In my own app, I have added the following intent filter to my voice-searchable Activity:

<intent-filter>
    <action android:name="com.google.android.gms.actions.SEARCH_ACTION" />
    <category android:name="android.intent.category.DEFAULT" />
    <category android:name="android.intent.category.VOICE" />
</intent-filter>

However, if I give the "OK Google, search for computers on [my app]", the voice category does not get added to the intent:

02-26 16:17:26.722 779-14786/? I/ActivityManager: START u0 {act=com.google.android.gms.actions.SEARCH_ACTION pkg=com.my.pkg cmp=com.my.pkg/.activity.VoiceSearchActivity (has extras)} from uid 10027 on display 0

Because this category is not set correctly in the Intent, Activity.isVoiceInteraction() and Activity.isVoiceInteractionRoot() are both returning false.

Can anyone explain why this might be happening?

Thanks!