The issue here might be caused by the framework cache. GetProperty<T> and GetProperty(AutomationProperty) behave differently for different controls, usually the generic method will do a look up just for the cached values for speed, so you can switch between the two if the first does not return the proper value.
I've written simple Silverlight test against our demo site but it should be the same for WPF apps.
var slApp =
var vls = slApp.Find.ByType(
var maxValue = vls.GetProperty(
that should work as per your case.