Not sure if it's the same but sounds similar.
I had an army with my Sovereign that included a Party of Blitzers (4 members, as Party options says) against a Troll. Battle was won apparently without casualties in my army. BUt after the battle I noticed that the Partyy said 3 instead of 4. I supposed that I had lost 1 men in the party. But some turns later I discovered that all my new parties were all of 3 men. Why?

Altough I have the save game (maaaany turns later though), I crashes when trying to load it so I don't know if it's usefull or not.