From the card and map addon marketplace page: Important Notice:
The Waterdeep: Dungeon of the Mad Mage - Map & Card Deck
Addon is not designed to be added into the core Dungeon of the Mad Mage
adventure module, as these elements already exist within the module.
Attempting to import them into the core module will result in your
current maps being overwritten by the addon's maps. Since the addon maps aren't populated, you replaced the populated maps with clear ones from the addon. You could try a rollback, but if that doesn't work, then you probably just need a fresh instance of the module. If you are very far into the module, you could create a new game with the module and use the transmogrifier to bring the correct (populated) maps into your current game. I'm not sure which would be quicker/easier.
