It seems to me that some interns code was added to the battery measurement app. When not charging, the battery gauge drops at a constant rate regardless of use. When charging, it goes up by a constate rate. If low power mode is disabled and not charging, the gauge dropping stops. Regardless of my observations, the fact remains that the battery measurement app doesnt report the true charge of the battery screencap of battery history - https://developer.apple.com/forums/content/attachment/518b7b9c-e4ca-4c14-bffb-ff94bcca7f37

A possible workaround is to enable low power mode. it seems to prevent the bad code that subtracts charge from the battery gauge regardless of use. However, the gauge may not reflect the actual charge since this bad code has corrupted the charge measurement. In the pic, you can see the charge in yellow remains about the same for over 8 hrs. Yellow is when low battery mode is on low bat mode screencap - https://developer.apple.com/forums/content/attachment/668e22b8-c603-40a6-9ff7-a701f3526c17