Microsoft will make an update to the To Do service on October 31, and advises To Do users to update mobile and desktop apps to the latest version for the best experience. Make sure your mobile and desktop To Do apps are version 2.49 or higher before the end of October 2021 to avoid any issues with cross device syncing. Also, since To Do tasks are linked with Outlook Android and iOS apps, you should make sure that you update your Outlook Android and iOS apps as well.
You can learn more about this update and what you need to do on Microsoft’s website.