Hello, everyone. It's about Light Legendary monster Bright.
What "Stellar Horns" skill do? Description says: "Deals moderate Light damage. Gives Damage Increase to all allies. Requires cooldown. Damage boost. Rounds 2"
It does damage to enemy and gives buff "Damage boost" to all allies. But in reality, damage does not increase.

Fixed Team Damage Boost - Now multiplies damage output by 1.5

Thank you for your report and sorry for the inconveniences caused!