I notice that sometimes there is no auto attack skill set (e.g. when equipping a weapon type you have not equipped before, when picking up a bundle). Try being the ‘heavy gunner’ type in the PvP snowball fight and you’ll see that every time you take the weapon out, u need to set an auto-attack even if u set one last time u had the weapon out.
Why not set skill 1 as the default auto attack skill?