I think it's a bug, and I have a similar issue with Byzantine Spearmen. At one of my castles where I've got the level 5 barracks, I'm supposed to be able to build Byzantine Spearmen with 3 XP. I've also got the Swordsmith's Guild HQ, for +2 global XP for knights. I can build Latinkon which come out with 2 XP fine, but if I build Byzantine Spearmen, they come out with nothing, and I have to retrain them to get the 3 XP.