It's intentional, upgrades would be ineffective otherwise.
For all doctrines except Allies the upgrade cost is half the production cost, upgrade time half the minimum production time of the target level. For Allies the upgrade cost is 20% lower.
This makes upgrading multiple levels up more effective. Basically if the health times damage (vs. any armour class) of the target level is atleast double the health*damage of the starting level it's worth it.


