For the record, if you haven't read the path notes, this is NOT a bug; ALL melee damage MODS have been decreased from +20% to +5%

Try using penetration mods instead for chain weapons, they'll be far more effective in comparison

You might still want to go melee damage mods for power weapons; there's not a lot that's going to have 140+ toughness so penetration mods will likely be redundant for power weapons.