I have the same issue. I bought my Surface Book 2 in mid 2018 and used it for office work at start. I used to have like 8h+ battery life at start, so I could use it at office without charging for a day.
However, since then I have been mostly using it at home and started to realize that the battery drains while it is totally shut down too. Today, I tried to start it after one week of shut down and the battery was nil. I couldn't start it without keeping
the charger connected for some time. Even after connecting the charger, the first time I started, I could log in but then suddenly battery died.
This is a serious issue and I would be grateful if someone from Microsoft can respond and say what they are going to do about it?