Song of Flame is eating surges, perks, Weapon stat bonuses and anything that would buff Solar effects applied by Weapon Damage while it's active. Weapon damage itself is buffed by its Radiant effect normally. Weapons like One Thousand Voices and Dragon's Breath are the most affected by this, since they either have bonuses to their Solar effects or are used to trigger Ignitions for build purposes. Dawn Chorus doesn't lose the 2x Ignition damage(most of the time), but the buffed value becomes the base for this bug.