Wisconsin doesn’t have party registrations as part of voter registration (we used to, back when I first registered, which is likely why there’s some partisan representation, but not for a long time), nor do you need to be registered with a party to vote in the primaries (everything is on one ballot, you can just only vote for one party).

So that huge chunk of “independent” is probably “new” registrations from the last 15 or so years. They just don’t have data because it stopped being a tracked metric.