Thanks again, to figure that out so precisely.
My observations, that a voltage coefficient is not recognizable for me, referred to directly connect 4s 18650 3400mAh NCR18650B (measured capacity 3200mAh) to the reference. So it tooks about 5 days, to go from 16.5V down to 13V - 12V, (not 9V as a difference), before i took them out to replace them with a freshly loaded set. With PX, I did it for 5 weeks. Before that some weeks with KX. With it i have more trouble with not so constant results, maybe because this is not an LTZ-A type, don't know at the moment.
But with both i didn't see any impact, that I could attribute to the declining voltage. In particular the buffered px shows a little tempco in the expected frame and meanwhile about 4µV voltagedrop, but can't and couldn't see this 11µV difference, that correlates to this 6V supply change.
I didn't made this precise measurements. So i think, the interpretation error lies with me. Maybe some tempdifferences (measured between 21 - 22°C) overlapped, or my PX react differently, don't know. But I believe you, there must be any supply voltage coefficient.
The time brings it to me.
Thanks for your effort.