Android : Background gradient on map marker custom drawable doesn't show up correctly on Android 4

I'm creating custom map marker icons that include an image and text. The text has a map_marker_glow.png background, which is an image that provides an oval-shaped white gradient behind the text so that it's still readable on the map.

It works great on Android 5:

However on Android 4 the gradient is showing up with a gray oval around it. Here is an image from 4.3:

Any ideas on why this is happening, and any code / image changes I can make to resolve it?

Here is MapMarkerIcon.axml:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:orientation="vertical">
    <ImageView
        android:id="@+id/map_marker_icon"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="center" />
    <TextView
        android:id="@+id/map_marker_text"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="center"
        android:gravity="center"
        android:textColor="@color/black"
        android:textSize="@dimen/text_size_count"
        android:textStyle="bold"
        android:paddingTop="5dp"
        android:paddingBottom="5dp"
        android:paddingLeft="10dp"
        android:paddingRight="10dp"
        android:background="@drawable/map_marker_glow" />
</LinearLayout>

And here is the code that creates a bitmap out of the layout, which can then be set as the map marker:

private Bitmap CreateIcon (Drawable drawable, string text) {
            EnsureInit ();

            // Update the icon
            iconView.SetImageDrawable (drawable);
            drawable.Dispose ();

            // Update the text
            textView.Text = text;
            textView.Visibility = !String.IsNullOrEmpty(text) ? ViewStates.Visible : ViewStates.Gone;
            if (text == " ")
                textView.SetBackgroundColor (Color.Transparent);

            // Lay out size and measure
            var measureSpec = ViewGroup.MeasureSpec.MakeMeasureSpec (0, MeasureSpecMode.Unspecified);
            container.Measure (measureSpec, measureSpec);
            int measuredWidth = container.MeasuredWidth;
            int measuredHeight = container.MeasuredHeight;
            container.Layout (0, 0, measuredWidth, measuredHeight);

            // Take current layout and draw to a bitmap
            var bitmap = Bitmap.CreateBitmap (measuredWidth, measuredHeight, Bitmap.Config.Argb4444); // According to API docs, this is deprecated to Argb8888; however, testing shows a reduction in memory pressure
            bitmap.EraseColor (Color.Transparent);
            using (var canvas = new Canvas (bitmap)) {
                container.Draw (canvas);
            }
            return bitmap;
        }

        private void EnsureInit () {
            if (container == null) {
                container = ApplicationContext.Activity.LayoutInflater.Inflate (Resource.Layout.MapMarkerIcon, null) as ViewGroup;
                iconView = container.FindViewById<ImageView> (Resource.Id.map_marker_icon);
                textView = container.FindViewById<TextView> (Resource.Id.map_marker_text);
            }
        }