Twice since the update I have fired off the Infernal King and it has reduced the top troops life to zero but it has remained showing. Once all of the remaining are killed the zeroed troop remains. Since life is at zero, further skull and magic damage do not effect it and there is no way to trigger the “win”. However, when it continues to match gems it does regular skull damage, however, it does not gather any more gems and cannot cast spells.
This is a known bug commonly known as the “Ghost Troop” bug. The devs are still trying to nail it down.
We do have a fix out on stream for this I believe. iOS and Android require a client update for the fix to come through.