Same issue, No mods, New Save (30 days in), No cheats. 1.6.0
Before quest = 17 Infantryman
Battle screen = 3 Infantryman participated in fight (all survived)
After Quest complete = Only 3 Infantryman in party
The quest is using a small part of my army for the quest 3 out of 17 infantryman, the rest are looters under the battle screen. Then after the fight only the 3 infantrymen used in the fight remain. The over 14 infantrymen who did not fight are wiped.