Hi Pieter,
I really appreciate your help.
Unfortunately, the function you describe is not mentioned in the A6100/A6400 user manuals; it only appears to be supported by the A1 camera body: https://helpguide.sony.net/ilc/2420/v1/en/contents/231h_preset_zoom_focus.html.
I will give it a try anyway.
Thank you and have a good day,
Paolo
Ah sorry, I missread your question as 'focus distance resets' rather than 'focal length resets'.
Apparently with a power zoom you can save your focal length to any of the Memory modes. With the camera then set to the relevant Memory mode, it should recall your last used focal length.