Thank you for confirming this. (was not in standby).
My initial guess was that it is just buggy code in the EC.
See my long investigation here:
But, with the laptop powered off, and the PSU cable removed, the EC is not running, so even buggy code in the EC cannot affect this.
I guess there might be an edge case, say if the EC has set the laptop to “DISCHARGE” just before shutdown and not left it in “NORMAL” state.
My FW16 does not show the problems you are seeing with the FW13, so I don’t think I will be able to reproduce your problem.
If I had the FW schematic, I could probably find out if the “DISCHARGE” state continues even if powered off.
Lastly, what FW say about the CMOS-clock might be true. As in, although a CMOS clock only tends to draw about microwatt, maybe they have a circuit around that, that saps the power unnecessarily. But, that’s all speculation, without a FW schematic.