Still waiting on this issue to be resolved.
In the meantime, I used this simple workaround: download & install the free SoundFlower driver (link below), pull up the SoundFlowerBed app to activate the panel, route the SoundFlower 2ch mix to your external headphones, and then in Cubase: switch your Device Setup/VST Audio System/ASIO Driver to SoundFlower 2ch. Ensure your VST Connections Outputs are also set to SoundFlower 2ch (which will likely happen by default), and you’re good to go.
SoundFlower download 
Rogue Amoeba | Soundflower