How are the Prussian infantry units meant to be upgrading? I have always assumed that once a unit gains enough experience, gets a complete "star" of experience, it will upgrade to the next infantry type and the amount of experience goes back to zero. The process then starts again, gaining experience and then the upgrade happens.
The reason I ask is that I have built a Prussian militia garrison. I left it with the training officer until both elements had a star of experience and then removed the training officer. I didn't then check on them for quite a lot of game turns. When I did I saw that the two elements had gone through several upgrades

They hadn't received any more training and hadn't had any combat experience, but they have continued to upgrade. One of the elements is now elite infantry troop type.
Is this WAD?
Should the upgrade from each troop type be just dependent on time or experience gained?
Or is it ye olde bugge?
Can post save if needed.