What you were expecting to happen, and what actually happened?
Opponent with auto-summon doesn’t disappear after it’s death, if there was no vacant place. It stays (visually) on field, even if it’s non-exist in game sense.
What are the steps to make it happen again?
1.1. All places are occupied, and troop in question stays at the first position.
1.2. Troop have the ability “summon troop on hit” (I believe, it was spiders for me, but I can be wrong).
I hit troop and killed him, and (spider) immediately takes it’s now vacant place.
After I destroyed this troop, previous, now long dead troop taken it’s place with characteristics that was valid just before it’s death! (with ready spell screen and life that was before my hit). Also, it was not real, as I discovered when tried to kill him with my lyre “attack and get a turn on kill” action. Lost a turn, huh.