I'm not sure if this is even a problem or not, but I noticed that shooting a Drum of the Wild with a mana spreader equipped with the phantom mana lens activates the drum, but the burst continues going on.
This first screenshot shows the path of the burst going through the Drum.

This second screenshot shows the point at which the burst starts to lose its mana, which is below the drum.

The only reason I can think of this being a problem is that you can essentially activate the drum of the wild at no mana cost if you harvest the burst.

Meh, no problem.

This is now a feature.

The Horn didn't cost any mana either for doing the same thing manually.