I know it’s a bit late, but for anyone having a similar problem:
I had the same issue: A white border was shown on pre-lollipop devices.
I solved it setting the cardPreventCornerOverlap to false
on your XML.
Like this:
<android.support.v7.widget.CardView
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:card_view="http://schemas.android.com/apk/res-auto"
android:layout_width="wrap_content"
android:layout_height="match_parent"
android:layout_marginRight="6dp"
card_view:cardPreventCornerOverlap="false">
<LinearLayout
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical">
<!-- some other views -->
</LinearLayout>
</android.support.v7.widget.CardView>