Currently, if one goes into the notification center to read a progress notification for a city, the notification includes a completion percentage for the city. However, if one visits the notification at a later point, the percentage may have updated to show your current completion percentage. For example, if I complete 5% more of the city in the screenshot below, then revisiting the notification would show me “45%” and not “40%”, even though I received a new notification for the 45%. Thus, in the end you end up with a lot of different update notifications in your notification center for the same city, that all have different dates you received them, but the same percentage.
If it is intentional that the percentage is updated, then I would suggest that all notifications for the same city are merged into a single notification. If not, then the notifications should be changed to show the completion percentage at the time of generation, not at the current time.
Moreover, there seems to be an issue with the code that sends out the emails for the notifications. I have recently received several progression emails that congratulate me on completing 0% of a city (see screenshot below). However, if I visit the website, my completion percentage is over 5% and the notification on the website shows 5%. These notifications seem to be sent in with city updates.