[FlexLayout] Can't Nest a FlexLayout in a Nested FlexLayout

Description

A FlexLayout inside a FlexLayout works fine, but another nested FlexLayout disappears.

Steps to Reproduce

Here's some XAML attempting to reproduce a "holy grail" layout:

   <FlexLayout Direction="Column">
        <Label Text="Header" />

        <FlexLayout FlexLayout.Grow="1">

            <FlexLayout Direction="Column">
                <Label Text="Nav #1" />
                <Label Text="Nav #2" />
                <Label Text="Nav #3" />
            </FlexLayout>

            <Label Text="Content"
                   FlexLayout.Grow="1" />

            <Label Text="Aside" />
        </FlexLayout>

        <Label Text="Footer" />
    </FlexLayout>

Expected Behavior

The FlexLayout with the three Label children should be visible.

Actual Behavior

But it's not there are all.

Basic Information

• Xamarin.Forms 3.0.0.354232-pre3
• Visual Studio 2017 version 15.6.4
• Platform Target Frameworks:
  • iOS: 11.3
  • Android: 8.1
  • UWP: 10.0.16299

Reproduction Link

Complete solution at https://github.com/xamarin/private-samples/tree/master/FlexLayoutIssue