It's not just a problem with 1940, though. This is happening with every World Series game I've played in this project.
For example: the Chicago Cubs beat the New York Yankees in game 7 of the 1943 World Series, which I just simulated. It was an exciting 16 inning game. There were only 21,981 people in attendance at Yankee Stadium.
In real life, the Yankees drew over 68,000 for all 3 of their home 1943 World Series games.
It's happened in every single World Series I've played in this project, from 1936 through (now) 1943. The attendance hovers around 22,000, regardless of the city, the fan support, or any other factor that should be considered.
I haven't checked this closely because I've been autoplaying the seasons one by one. However, I also suspect that the attendance spread is off. Tight pennant races don't seem to produce sellout or near-sellout crowds in OOTP as they did in real life.
And, yes, Crosley Field was overfilled in 1940. That was actually pretty common for World Series games at the time, as well as games during the pennant stretch. You should also keep in mind that the capacity listed online is not necessarily a reflection of reality. For example, over 46,000 people attended a regular season Cubs - Cardinals game in 1931 despite the fact that Sportsman's Park seated only a bit over 34,000;
I wrote about this here, including original newspaper articles.
Now, that can be really hard to model, especially when you realize that it came during the worst days of the Great Depression, and when you realize that the Cardinals actually had a substantial lead in the league at the time. However, OOTP should at least be capable of modeling a sellout or a near-sellout for a World Series game.