The solution for non-100% finished campaigns

[>me bee]

Me, along with some other players, have found out that finishing the campaign ends with it not being at 100%.
In case you haven't figured out why is this, it's because you have not completed some covert missions, which are shown after unlocking specific bros.

So basically you'd need to finish a campaign once (or unlock a lot of bros around the start of the game), start a new campaign, and finish all the covert missions to get a 100% on a save slot.

Hope this helps to some people.