It may be too early to tell yet, but Microsoft sent out Build 18990 and came with the newer Firmware built in. It has only been less than two days but with testing, have not had a power drop at all. In fact, ran the machine (Surface Laptop 1st gen) for
a little over an hour with general web browsing and email and it only dropped to 93%.
Not sure if the Firmware is experimental and if is, not available for general download for others.
Here is the Firmware that was installed:

Good to know you are testing James 👍, but for current production machines facing the drainage issue, the fix for now is to turn off SMT via UEFI / Firmware Settings, build 1903 - 18362.356 - And from your description it seems back to the baseline norm,
As conflict with current firmware does not handle super scaler well. disable Simultaneous Multithreading (SMT) has units that I have tested operating normally as should.
in my case, with surface pro 4, the deactivation of Simultaneous Multithreading (SMT) does not reduce the battery draining (which I have after the August 2019 update, during normal operation and not when the PC is off). From what I understand, it depends
on the "Surface Embedded Controller Firmware - Firmware 103.2614.257.0 improves battery performance" update. I hope that a new firmware update will also be released for surface pro 4.
josèterra Surface Embedded Controller Firmware - Firmware 103.2614.257.0 is avail. for all Surface Pro machines download the
drivers at the following Surface Pro 4 Drivers & Firmware - Download the appropriate version for your
machine.
Once everything is up2date, drain the battery completely and charge until 100% full and test.
*For a visual understanding with Data Insights. Look below.

Prior to turning off (SMT)
