gap in the Basic Impetus Rules?

Yesterday I was platino a game to test the Rules and I'm not sure is there is a gap in the Rules.

One unit charge another unit in the rear but it was unlucky and the combat was a draw, so enemy unit is not destroyed and combat continue in the following turn.
In the turn of the enemy I see (with a lot of surprise) that tris enemy unit has no opción to face about combat and the only one things that can be do is fighting in a disaventage posición until their eliminación.

I was check the Rules agan, forums and FAQs and Can't see anthing about this situación.

The Rules seems funny and well constructed, so I'm surprise with this situation that other ganes systems solve in a logical way.

So, Is there a gap in the Rules?

Thanks,
Juan Miguel