Not a bug.Please read the card text carefully.Two conditions need to be satisfied before Warden can damage creatures with Airborne.1. The targeted creature must have its Airborne passive removed by Web2. The Warden using Guard must have more than 0 attack
I tested the behaviour of the warden + spider + atk against AI0 in several cases (SoP, Chaos Power , warden upped vs unupped). I find no bug. Maybe your case is a very special one, or you made a mistake. Screenshots would be helpful.