I'll admit I haven't actually tested this in game but bear with me.

In 0.982 you added several ways to specifically damage champions, such as weapons and traits that deal extra damage to them. To make this possible you added <CreatureType>Champion</CreatureType> to all champions and sovereigns. However, the XML was not added to Henchmen.

So is a Henchman a champion or not? It is immortal like champions, it has its own inventory like champions, it gets to choose traits when it levels up. I say they are champions. And therefore, they should be treated as champions with the added tag <CreatureType>Champion</CreatureType>. Otherwise, weapons and traits that specifically target champions will not function against Henchmen.