Proper parks were assigned in my v25 game when I started a new MLB game.
FWIW I import my game into each new version, I know not what is being discussed but I'll get to it.
On import all parks are correct. The only issue being if a park had a name change. In those cases I would get the "default" ballpark that I could fix.
On to starting a new MLB default season in v25...
I did start a current MLB game for two reasons and playing games wasn't one of them. I wanted to check out default uniforms and number/name files to see what was new before moving on with my imported game.
I also wanted to check out the new park factors in v25 so I could see them before importing and changing the park factors in my on going game. As I read this thread I thought "I'm pretty sure there was no issue in my game with park assignments". I just loaded up that league and confirmed all MLB teams are in the correct park and the correct model loads. Nothing had to be changed in my game.
This is with the Steam version on PC.
The only caveats are that no games have been played in my test league. But when I go to settings>edit ballpark for any team it takes me to the 2d page in the ballpark editor, with the proper graphic showing (ie Wrigley for Cubs, Fenway for Boston etc.). If I go to the 3d tab the proper stadium loads for each team. I can't see how this would show on the 3d tab but revert to the default stadium because I was playing in classic 3d? But then I haven't played any games.
Not sure why mine is different?
Or maybe I'm not understanding what you are seeing in your games? In any case thought it worth posting that default park assignments are not as issue in my game, for whatever that is worth.