When you create a flex container only the child elements become flex items. Descendants beyond the children do not become flex items and flex properties don't apply to them.
Simply apply display: flex to the flex item, which converts it into a flex container, as well. Then default flex properties like align-items: stretch will apply to the children (now flex items).
You wrote:
I would like to have a div container that also uses its parent's full
height...
You don't need to use height: 100% or add align-items: stretch (it's a default rule). Simply add display: flex to .flexbox-child, and .flexbox-grand-child will expand the full available height.
Modified demo: http://plnkr.co/edit/n0Wt3x3CUr1ZfBD2RrGo?p=preview
re: height: 100% possible bug
With regard to the need to specify height: 100% on child elements, I don't see any bug here. Everything seems to conform to the spec. Here's a complete explanation: Working with the CSS height property and percentage values
