Two factors seem to be affecting battery depletion: use of the mobile phone radio, but only sometimes; engaging with an application in a way that requires scrolling.
Yesterday I tried an experiment: I started with the phone charged to 100%. I opened the Chrome web browser and Lirum Device Info Lite. I visited the Techbargains web site, which is one of those endless scrolling type web sites, and started scrolling. Within one minute, then battery level was 84%. Jumping to Lirum Device Info showed the CPU use was unremarkable, not more than about 25%. Next, I closed the browser app and opened a solitaire game and used the game for about half an hour, which involved a lot of screen tapping and dragging card bit maps around on the screen, screen on all the time at 50% brightness. At the end, the battery was at 79%, which is about what is expected. Scrolling in the Apple Settings app also makes the battery drop quickly. I alternated between Settings and Chrome until the battery was at 11%. Next, I started the YouTube application and let it run until the iPhone 6S shut off. YouTube ran for over an hour. All the testing was done while connected to Wi-Fi and not using 4G data.
i am starting to suspect there is a combination of the battery health being over optimistically estimated and some change that occurred with the way the GPU is handled, especially when it is manipulating memory to scroll the screen. It is possible this was a latent problem, only exposed by the 13.3.1 update, or the update itself is causing the problem. We will have to wait to see.