Questions about the gameplay of Heroes 6.

Something strange happened in an online duel... I was playing as Necropolis (mage) against Haven (warrior). I cast "Time Stop" on the archers for 3 turns, but it wore off very quickly, even before their turn. I was completely surprised. I started reviewing the log – after the entry about my hero casting the spell and until the current turn, there was no mention of it being removed. No dispel was used. Maybe it's a cheat or a bug... but I don't understand how it could have disappeared.