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Under investigation by the game team
Any troops that have a chance to summon or resurrect another troop on death on the enemy's team can spawn over a Treasure Gnome causing them to disappear, for example Drake Rider, Incubus or The Dragon Soul.
Fixing the problem
Please contact support if you encounter this problem and mention the troops used on the enemy team. Please record a video or take a screenshot/image of the problem occurring and the battle rewards screen where possible.