It seems the TaskDialog doesn't support tab characters. For example,
TaskDialog taskDlg = new()
{
WindowTitle = "Test",
Content = "Lorem ipsum dolor sit amet,"
+ "\n\tconsectetur adipiscing elit."
+ "\n\t\tAliquam erat volutpat."
};
taskDlg.Buttons.Add(new TaskDialogButton(ButtonType.Ok));
taskDlg.ShowDialog();
shows the following:

WinForms MessageBox renders the tabs correctly:

It seems the
TaskDialogdoesn't support tab characters. For example,shows the following:

WinForms

MessageBoxrenders the tabs correctly: