Summary: After an event fight 12 archers were vanished.
According to description troops should not suffer casualties or injuries, however 12 archers were missing after this fight. Started with 81 and got 69 after fight. If you lose entire legion it is about 4 casualties in mine case so 3x4=12. Looks like in my case casualties were counted. Fight took place in allayed territory.
Incident time 13:22 EST (18:22 UTC) player ID: 2834411 server 82.