With the latest update to Age II: DE, a change was made that affects creators publishing mods via the Steam version of the game. Where your mods were previously published using your Microsoft / Xbox Live account, it is now using your Steam account. Your previously published mods are still available, you’re just not able to update them from in the game.
We are looking into this issue. In the meantime, if you need to update your mod you can log on to www.ageofempires.com with your Microsoft / Xbox Live account and publish your mod there. Or if you happen to also own the game via the Microsoft Store, you can publish from in that version of the game.