I've had similar issues with multiple champions when they are defending city with sovereign where they would resist all attacks with bows (1 champion without any decent stats decimated 7 leveled up champions with bows ) . I would usually snipe the sovereign and retreat. Then after retreating immediately come back and I would see the sovereign alone after it revives, kill and I would just capture city and never see that champion again... So there's also a bug in how garrisoned stacks are handled after retreating.