I have also encountered the exact same issue.
1. Recruited Wife into my army. She has 165 troops to begin with before the siege.
2. Joined Allied Empire culture to a siege better that they had initiated first.
3. Tried both Join the Fight and Send Armies options.
4. Despite only 3 - 5 losses and with 0 desertion, after the battle, either I moved a couple of distance in world map or opening the UI functions, my Wife's army suddenly roughly halved.
5. I have tried just having 25 troops for my wife's party and after the siege battle 1 troop disappeared, not died nor deserted.
6. I have tried placing all 165 troops in my town garrison and it is safe in there.
Image 1 (The numbers do not add up, it shows 331 on the left red circle while 407 in the right red circle.)
Image 2 (Right Red circle numbers updated after moving a couple of distance in world map.)
After much blood, sweat and tears. I found the culprit!
@MArdA TaleWorlds Please take note of this issue of parties doing whatever they like. Need to code them to be more obedient.
