Try assigning "InstructionText" in .Opend even once again (seems to fix the problem in 1.1.0.0 as well).
var dialog = new TaskDialog
{
Caption = Title,
InstructionText = "Some files added are already in the collection.",
Text = "They have been skipped."
};
dialog.Opened += (s1, e1) =>
{
dialog.Icon = TaskDialogStandardIcon.Warning;
dialog.InstructionText = dialog.InstructionText; // < seems to work
};