I've had forms that have second attack. When you fight in form, your second attack skill goes up. This suggests that your skill in second attack (out of form) affects your second attack results in form. All good so far (if this is correct).

Now, I recently got a form that lashes. I have a lash skill as a shapeshifter, so I'd expect this to work like second attack. But, my lash skill has never improved while lashing in form. Also, in form, lash almost never works. Could this be a bug?

I think that if a form has second attack, it syncs up because it's an passive ability not an active ability, something you must input commands to use. Because of that, lash wouldn't work for you in the same way it would second attack and is reliant on how comfortable you are with the form > how well you know lash outside of form.