Now this I see as an issue. Not all notebooks are the same, and I suspect that even in power down mode some part of the machine is still active. Depends on how it is made. In the old days desktop used small internal batteries to keep certain functions going, the clock for example. Notebooks rely on their built in batteries for these types of things.
I suspect part of the issue here is related to the release mechanism, which apparently stays active even when powered off. Remember it is an electronic release not a mechanical one. Microsoft most likely did this to prevent people from breaking the device by releasing it when powered off. Remember most two and ones are mechanical not electronic. I can live with a 4% drop over two days. That is not an issue for me. Before it was losing about 10 to 15 percent a day, so the update has helped me.
I don't think that you can expect it to perform like other two and ones or notebooks that are far less complicated.