Old bug really, not the 1st game this exact problem is happening. But previous cases (with other games) were fixed on your end (globally, was never my account problem).
So there is a list of DLC on games page in Ubisoft Connect PC client. It is supposed to show what DLCs you own and what you can buy. I have season pass and its displayed as owned. But "wrath of the druids" DLC is offered for purchase, it doesnt have a green title with checkmark. Meanwhile Connect properly downloaded and installed that DLC without additional purchase, and it is available inside the game. It is also listed in account information. Only DLC list on game's page is out of sync with everything else. My SKU's name (taken from store confirmation email) is "PC DIG-ASSASSIN'S CREED KINGDOMS-ULTIMATE-WW" if it matters.
Still not fixed and I now also have exact same problem with "Siege of Paris".
Get your bling together, Ubisoft. Yours is the only digital game store I use or used that cant even keep track of owned products properly. The only digital store that lets you buy the same product over and over again (aside from gifting that you still dont even have). Im starting to think you actually fish for those "sales" on purpose.