Saying that tracking (toe) doesn't change with wheel size is a little misleading.
If a car is setup for 17" rims and then you put on 19" rims , the toe will be out enough to cause excessive wear.
For example if you put a 0.05 degree toe onto the car this will equal 0.754mm of toe on 17" rims and 0.842mm on 19".
Curious thing is that (if Alfa preset geometry prior to wheels being selected) this example would cause 19" tyres to wear less on the inner edges than 17" ones , not more. This logically leads me to believe that Alfa setup all the cars to parallel and leave the dealers to setup geometry as part of the PDQ checks.
In short , whatever the reason is , get your Alfa checked for correct alignment when you buy it (new or secondhand).