getAllTrackables(AugmentedFace.class) tracks a single face

SPECIFIC ISSUE ENCOUNTERED

Using the following code for getting a list of augmented faced from the camera:

Collection<AugmentedFace> faceList = sceneView.getSession().getAllTrackables(AugmentedFace.class);
Log.i("facetracking", "detected: " + faceList.size());

I am consistently only getting 0 or 1 face detected, as output logs have indicated:

2019-12-30 23:05:48.829 10685-10685/com.google.ar.sceneform.samples.augmentedfaces I/facetracking: detected: 1
2019-12-30 23:05:48.880 10685-10685/com.google.ar.sceneform.samples.augmentedfaces I/facetracking: detected: 1
2019-12-30 23:05:48.945 10685-10685/com.google.ar.sceneform.samples.augmentedfaces I/facetracking: detected: 0
2019-12-30 23:05:49.014 10685-10685/com.google.ar.sceneform.samples.augmentedfaces I/facetracking: detected: 0
2019-12-30 23:05:49.081 10685-10685/com.google.ar.sceneform.samples.augmentedfaces I/facetracking: detected: 1
2019-12-30 23:05:49.130 10685-10685/com.google.ar.sceneform.samples.augmentedfaces I/facetracking: detected: 1

This is despite having two faces in frame.

In fact, if I cover one face, tracking immediately swaps over to the second face, and the initial face is subsequently ignored until the second face is covered.

As per the sample code, I have a section for adding faces as they appear ready for tracking:

for (AugmentedFace face : faceList) {
    if (!faceNodeMap.containsKey(face)) {
    Log.i("facetracking", "found: " + face.toString() + " " + face.getTrackingState());
    AugmentedFaceNode faceNode = new AugmentedFaceNode(face);
    ...
}

It appears that the conditional is only satisfied once, resulting in the following log message appearing once:

2019-12-30 23:04:54.598 10685-10685/com.google.ar.sceneform.samples.augmentedfaces I/facetracking: found: com.google.ar.core.AugmentedFace@3ea1b78 TRACKING

I also have this as the code to stop tracking:

while (iter.hasNext()) {
    Map.Entry<AugmentedFace, AugmentedFaceNode> entry = iter.next();
    AugmentedFace face = entry.getKey();
    if (face.getTrackingState() == TrackingState.STOPPED) {
        Log.i("facetracking", "lost: " + face.toString());
        ...
    }
}

Which doesn't print to the log at all, even when I've clearly put my phone down and I get repeated "detected: 0" log messages.

TL;DR: Getting all trackable faces is really only giving my one, and that face is seeming always in a TRACKING state.

VERSIONS USED

Android Studio: 3.5.2
ARCore SDK for Android:
Device manufacturer, model, and O/S: OnePlus, OnePlus 6, Android 10 - OxygenOS 10.3.0
ARCore: 1.14.191118106 On Windows, use: adb shell pm dump com.google.ar.core | findstr /i "packages: versionName" On macOS, use: adb shell pm dump com.google.ar.core | egrep -i versionName\|packages:
Output of adb shell getprop ro.build.fingerprint: `OnePlus/OnePlus6/OnePlus6:10/QKQ1.190716.003/1912142009:user/release-keys

STEPS TO REPRODUCE THE ISSUE

Add the log outputs to the sample augmented faces app
Run the app with multiple faces in frame
Individually cover each face and observe log output (or run a debugger)

WORKAROUNDS (IF ANY)

n/a

ADDITIONAL COMMENTS

Is there perhaps some documentation I've missed marking this as a limitation? The examples and API suggest that multiple faces were meant to be detected, although clearly that isn't the actual behavior in this case.

google-ar / arcore-android-sdk