I created a support ticket; I'm copying here in case anyone else has experienced this and maybe has better info.
Does the Quickfire ability nerf damage?
The next battle after I selected the Quickfire ability (and every battle I've fought since), my hero has been doing very little damage. Before, he was doing 14-16 damage, and could wipe out weak groups of enemies. Now he does 1-2 damage and is basically useless.