I’ve had the same problem since the beginning. I think it it the hit box of the boss is to high if you are sitting, the orbs you fire will fire depending on your height, so, if you are sitting your orbs will fly underneath the bosses hit box.
The problem is occurring to me on almost all bosses
a fix would be just to make the hit box of all bosses (and normal mobs) go all the way to the ground, or make the bard orbs be at a specific height for it to reach the hit box.