Just a follow-up to say that all seems to be working using my Sesto Senso 2. I have upgraded to 3.6.2, and tonight had some clear weather to test. I turned off 'AF backlash comp' in the autofocus panel, and added my backlash setting for the focuser in the INDI driver. The Linear 1 Pass now shows perfect curve-fits and the solutions correspond with the curve and the actual focuser position.
I really like this autofocus algorithm, so very happy that it now all works. Thank you for your persistence in finding a fix, John.