Thanks @Doug and @Peter,

@Doug, the presented V-curve is not the most weird, look please at the one below. I began at 7715 and finished at 7715, the HFR is even better, but the shape is not. Can you try to explain this behaviour?
I'll try the 7% next time and then will also try to shorten exposures with higher ISO value. Now I used 15s at ISO16000 (IDAS LPS-P2 filter), I'd like to drop it to 10s or less, because it takes time as well.

@Peter, I cannot find the Linear 1Pass. I have only the following algorithms on the list: Iterative, Polynomial and Linear. Is the Linear 1Pass algorithm introduced in the most recent version?
I know that the polynomial algorithm doesn't take the backlash into account by default, but I don't know how to measure it. It must be a long cloudless night to test everything, without the rush and risk that darks or flats may fail because of sudden rain. I expected the rain at 5am, thankfully a tree appeared in the frame, so I stopped doing subs and finished at 3am. God, my location teaches me humility...

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