Same thing happened to me.
I cast Mythos of Illuna, with the additional fight effect, and copied the opponent's 5/5 creature.
I then used Startle to reduce the opponent's creature's power after copying with Mythos of Illuna, but the decayed zombie did the subsequent fight instead of the Mythos of Illuna token. This lost me a game due to the fight not removing the opponent's creature.
Same thing happened to me.
I cast Mythos of Illuna, with the additional fight effect, and copied the opponent's 5/5 creature.
I then used Startle to reduce the opponent's creature's power after copying with Mythos of Illuna, but the decayed zombie did the subsequent fight instead of the Mythos of Illuna token. This lost me a game due to the fight not removing the opponent's creature.