Shipping Promo spanning multiple shipping strategies

Hi,
Let’s say you have a different strategy for 750ml and 375 ml.
You create a promo that applies to “ground” within every shipping strategy - 1¢ flat rate ground.
Customer shops for 1 x 375ml and 1 x 750ml.
Promo applies itself, but the shipping is $0.

is that a bug? Or you are supposed to do 2 promos, 1 for each shipping strategy?