OK! This was actually what I suspected. You've got pretty good evidence there. :-) Maybe someone should tell/ask the city why their service indicates what it does so they can fix their source data as well - if there's something to fix. In the meantime i suggest we change the housenumber of thd building polygon to "Mäntykatu 1" and describe the situation in detail behind the description tag but also add a address node for "Pitkäkatu 32" to indicate that at least some data might also use that address.