The new prefix shard “Sinathia’s Dying Breath Reforged” has two components: flat added Curse Damage and % increased Curse Damage. The flat + Curse Damage does not work and will not increase any curse damage at all (anguish, torment, decrepify, etc.)
Tested it against a helmet with [+ Spell Damage for Curses] affix. The flat damage from the helmet actually works, and curse deal more damage to test dummy. But Sinathia Reforged mace does nothing, even taking account the % damage increase.