I entered the industrial era and played a bit after. I can't think of any specific action that jumped out. I saved the game, shutdown. Came back to my computer later and booted my game up. Now my map is dotted with these gray tiles. It appears the tiles are where districts should be. I then tried reloading, verifying integrity of the steam cache, relaunching the game, nothing worked.

I have the same issue, where some tiles are grey are these grey blotches, and some show as empty terrain and do not display the district/wonder that's built on them. It also appeared for me in the industrial era, upon the first save load after changing from the Ottomans to the Siamese. Certain tiles that display this include water improvements, non-farmer's district farmland, and the St. Basil's Cathedral wonder. Reloading to a save in the early modern era shows the same tiles/improvements correctly.

I'm running the game from Game Pass for PC (as I understand it, essentially the Windows Store version), on Windows 10 Pro 20H2, with a Ryzen 5 3600, RTX 3070 (driver 471.11), and the game installed to SSD.

As expected updating to the latest graphics driver, 471.68, had no effect.