Which version did you start the game with? Do you have a chance to load up older save games and check the status of the hex trying to figure out which turn it changed from Yugoslavian to Russian?
When I start new games then the hex is Yugoslavian. It remains Yugoslavian when Barbarossa starts.
Is it only Split that's Russian or any other hexes in Yugoslavia?
The game code was checking for core hex owner regarding who should own a hex. The game doesn’t have a core owner for a port because it’s controlled by the adjacent city. So the test failed for port hexes because it believed that the port was controlled by Yugoslavia as non-Axis and it was not a core Yugoslavian hex. Then the owner will be set to the controlling major power. That is Russia for Yugoslavia if Russia is activated. The code is changed so it only checks land hexes. The adjacent port will change owner to the controlling city as it should.