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: