That's because the game is broken, you can get absurd amounts of AP in just a few minutes.
The bug? Don't buy upgrades, attack a country and then buy ONE upgrade, instead of adding a fixed amount, it will be multiplied, if you repeat this with all the countries you will obtain an absurd numbers. Then you can buy a single upgrade for each unit, all of them will go to more than 1e20.
Another bug is that the auto stops working and I'm not sure but I think it's when you buy an upgrade at the moment the auto activates. If I buy other auto it fixes itself.