ToolBar: ToolBarTray doesn't respect ToolBar's HorizontalAlignment property.
ToolBarTray is actually a Grid supporting Band and BandIndex properties of the RadToolBar.
To simply position RadToolBars with Horizontal/VerticalAlignment properties you can use Grid instead of RadToolBarTray.
The XAML team has recently reviewed this issue and will not be addressing it as at this time the team is focusing on the bugs impacting the highest number of developers. If you have encountered this issue and it is blocking for your work please contact us through the support ticketing system with details on your setup and a reference to this item.