This is a fairly odd one, but it appears that 100% of the time: the first time "Sample Data" that is generated for a newly-built Data Set, the second item in the array will always fail to fill its Value field.

The first time I saw this I thought it was an unlucky roll of the dice, but it kept on happening over and over again every time I created new Design-Time Data in a new Project, completely defying the odds of Random Number Generation.

Repro Steps:

1) Create a Studio Project featuring a MainPage.xaml

2) From the Data Panel, create a DataStructure named "MyStructure", containing a DataProperty of name "MyString", of type 'String', and set to be served as a 'Single'.

3) From the Data Panel, create a second DataStructure named "MyViewModel" containing a DataProperty of name "MyCollection" and of type "MyStructure" (referencing the Structure created in Step 2), and set to be served as a 'Collection'.

4) From the Data Panel, create a Data Set called "MyDataSet' from the "MyViewModel" Structure, apply it to the Page.

5) Press the 'Generate Sample Data' button, then expand all items in the array to reveal their Data Values.

Observe how the second item (Number "1" in programmer counting terms), has no value filled out.

6) Create a new Studio Project, and repeat Steps 1) to 5) again.

Observe how the same phenomenon occurs again.

7) Repeat in fresh Projects as many times as desired.

Observed how the same pattern occurs over and over again.

Expected Results: The 'Generate Sample Data' is truly randomized and doesn't contain a second-item bias to not fill.