When the attack was done both players were on the same version. One of the players - either you or your opponent - updated after the raid ended. Once you’re both on the same version you can revenge the raid.
That being said, this happens each time there is an update to the game. This time was especially noticeable as there were 2 updates within a couple of days. That gives 3 possible versions so is more noticeable. Good news is everyone should be updated soon 