The battery draining while the dGPU is in use on the 15" Surface Book 2 is a hardware issue where the power cord simply can't provide enough power to the machine. I think this was an issue with early cords. At least one review of the Surface Book 2 noted that when Microsoft replaced their review unit's power cord, the issue mostly went away.
https://www.theverge.com/2017/11/16/16658466/microsoft-surface-book-2-review
I have a 13" SB2 which has a Nvidia 1050 instead so I have no power issues. But I can see how the combination of the power cord issue with the v1903 disconnect issue makes for an extremely frustrating experience.