In sense fields that can utilise functions have FX to the right on the input box. I.e. On the appearance->title property you will see FX. Therefore this label can evaluate a function. However the label for measures does not have the FX indicator. Therefore it won't be evaluated.
If you do this in expression it will show in the selections bar the full expression , dont know the reason behind this particular behavior in Qlik, for all the fields which you are doing calculation in expression even you give label in selection bar the label will not show.