Z height inconsistency  

Z height inconsistency

Hello everybody, I'm new in this forum. I would like to thanks the Prusa team for the great Job. THANKS!!!!!!!! <3
I bought the Prusa i3 mk3 a few months ago. Since then everything works great and I had a great 3d print experience.
A couple of weeks ago I bought the carbon filament and the hardened nozzle. I printed some very wonderful work with that filament.
A few days later I decided to unload the carbon filament and load the pla filament to test the hardened nozzle behavior with the common filament.
When I launch the print the nozzle crashed on the bed surface trying to print at Z=0.
My PEI sheet was ruined and I switched the hardened nozzle with the brass original one and I did all the calibration again.
It was very hard to find a good setting because the right side of the bed was higher than the left side. But after many tests and after I used the bed compensation settings I reached a good first layer setting.
After that, I restarted to print my stuff for a couple of days. But without any reason, the printer after the bed leveling procedure starts to print without increase the z height so the nozzle crash on the PEI bed with damages and discouragement.
I tryed to downgrade the firmware to the previous version but nothing.
I tried to print gcode file from the original prusa SD card with no results.
I can't understand what's the problem.
I hope someone in the community can help me.
Thanks to anybody for any help.

Bruno