It's down in the XML that the maximum XP after upgrade is 10.
It kind of makes sense, your guys were very good with their axes, but you've just put different weapons in their hands- so they need to retrain.
You also didnt lose their promotions, just the nearness to their next one.
That is how the game is supposed to operate- there's no bug.
However, you could just edit the XML and set that "maximum XP after upgrade" value to 9999 or something if you like that better.