Unfortunately I don't know how to fix it, but it looks very much like what I am experiencing: general-discussion-announcements-and-re ... 5752.html
I think it is a bug in the software... (or I don't understand the live Z settings, but then I'd appreciate an explanation).
In my case, the ever changing Z height occurred after I had played around with OctoPrint (mostly to visualize the height differences between the 9 PINDA measurements because I tried to increase flatness by adding very thin spacers under the mounting screws).
I don't know whether the Z height issue was really related to using OctoPrint or whether its occurrence at that time was only mere coincidence.
Point is, Z height increased from print to print afterwards even with no OctoPrint connected. A cold boot reset the Z height back to what it should have been, but after that it kept increasing again from print to print. The change in Z height looked as if it the correction made affected the new reference level in the next print. (e.g. a working live Z went from -0.7 to -1.4 and then -2.1).
In the meanwhile, I have re-run the "Wizard" in the menu to "reset" all settings, and Z values are stable again (but I did not re-connect OctoPrint).
Would love to hear from some of the Prusa folks whether this problem is known...