Tail Whip bug

I was doing a Solo battle. I attacked using Vulpix' Tail Whip and got a heads, so the opponent shouldn't be able to attack.

Flip a coin. If heads, the Defending Pokémon can't attack during your opponent's next turn.

However the AI simply evolved their Pokeémon and attacked. I don't know if this is how it's supposed to work but it doesn't seem to be right.