I originally bought the base game on an alt account and used Family Sharing to play it on my main account. All subsequent DLCs were also purchased on that alt account. However, the Dawn of the Dukes DLC was given for free at one point, so both of my accounts own it.
I don’t know why but somehow this caused two positives to become a negative. This DLC’s campaigns appear locked on my main account, and is only accessible when I use the alt account.