Hi Sampsa,
First, this behaviour has nothing to do with the List containing a Structure, Lists of simple types should behave the same. Secondly, whether Lists are passed by reference or not depends on a lot of factors. On Web, a List is passed by reference if the Action called is in the same eSpace or in an Extension, and there is no conversion (implicitly or explicitly by mapping). Otherwise a copy is made. This is all very unclear, and imho OutSystems should change this behaviour, but it would be a breaking change, so they probably won't. Apparently on Mobile they made the right decision and always create a copy.