one might as well count Beethoven as Dutch or Stravinsky as Ukrainian
Beethoven's family stemmed from Mechelen, present-day Belgium, 'Austrian' Netherlands at the time (and never 'Dutch'). The point of course is, that it was long before the 19th Century created 'nationality'.
What I now learn, is that the son, Roine Rautio (1934-1961), stemmed not from Karelia, but from East Karelia, Petrozavodsk, which was never part of Finland. In that case it's highly unlikely he ever held Finnish nationality.
some history from Wiki: During the Finnish occupation of East Karelia in the Continuation War (1941–1944), the occupier chose to style the city Äänislinna (or Ääneslinna), rather than the traditional Petroskoi. The new name was a literal translation of Onegaborg, the name of a settlement marked on a 16th-century map by Abraham Ortelius near the present-day city, Ääninen being the Finnish toponym for Lake Onega.
The city was occupied by Finnish troops for nearly three years before it was retaken by Soviet forces on June 28, 1944. The Finns set up internment camps for civilians of Russian ethnicity which they operated until the Red Army liberated the area. Six camps were set up in Petrozavodsk, with 23,984 civilians of Russian ethnicity confined in them. Civilians of Finnish, Karelian or other Finnic descent were not interned into these camps. Some of the camps were old Soviet camps and some only fenced city areas.
In 1977, Petrozavodsk was the epicenter of what is called the Petrozavodsk phenomenon.