turn 6 Edit: In case you didn't know, the evasion clause bans double team and minimize and has no effect in-battle.
This was a Showdown! bug and has been fixed. source
Okay, it took me some time, but my cousin made a video showing that Defog will not fail then used in a battle. Also, since you were on showdown, the most likely reason is that there was no fog to clear. This is the video. Sorry that it cuts off late and has some glare.