vaadin-combo-box not displaying all values upon initial dropdown

I'm using Polymer with a vaadin-combo-box. And given the below stripped-down snippet of code...only the first three values show upon initial drop-down. I do see a blank spacing for the
remaining values, but only the first 3 values show up. Upon any subsequent dropdowns, I see ALL values just fine. Has anyone ever experienced this and/or know what the problem might be?
It seems to behave the same in Chrome and Firefox.

My code example:
<dom-module id="ecm-vaadin">
<template>
<style>
</style>
<vaadin-combo-box label="Element"
items='["Cadmium","Caesium","Calcium","Californium","Carbon","Scandium"]'>
</vaadin-combo-box>
</template>
<script>
Polymer({
is: 'ecm-vaadin'
});
</script>
</dom-module>