This is indeed the expected behavior.
The pick and drop mecanism of the EiffelStudio's editor is not "transporting" the qualified call but only the class or feature stone. (And more generally the Pick and drop mecanism of the editor does not take into account the selected text, but only the pointed token)

That's is to say, when you pick 'f' from 'a.f', the pebble (stone) contains a few information including class data of f, and feature data of 'f'.
When you drop this 'f' pebble into the context tool for instance, it will open the feature tool on routine 'f', the same occurs with the watch tool.

If you want to add the selected text as expression, you must select your text 'a.f' and left click on the 'Create a new expression' button of the watch tool.