Hi Lionsmane,
Unfortunately, this is a bug.
It's just a visual issue until you run into the insufficient population portion and then it becomes more of a major issue. If you have the required population (even though it is A LOT more than what it should be), you can still upgrade and it will only cost the regular amount (according to wiki). However, when you do not have the sufficient population (even though it is incorrect), it will not allow you to upgrade.
We have reported this with a "major" classification, I will update as I have more information. Sorry!