Thanks for getting back to us.
Have done a little digging ourselves and it looks like we spotted the problem.
We were initializing our view and then binding to it then trying to set the selected item property, but at this stage the gallery has not yet loaded the brushes bound to its CategorizesItemsSource property. We expected this would happen when the gallery was initialized, however at that time the gallery has no items, and so our selection setting fails.
Due to time constraints we can't create a sample project for you, however if you have any thoughts/tips that may help we'd appreciate it.