I have got another problem similar to this one:PrintTask.PrintAsync: Features don't disappear
BTW, thanks for the prompt and good answer to the above one! It works for me.
We are showing labels on a FeatureLayer by adding AttributeLabelClass to FeatureLayer.Labeling.LabelsClasses.
This works fine, Labels are shown on the MapView.
But when using PrintTask.PrintAsync the labels are not shown on pdf, gif etc. document.