Marlin 2.0.9.3 leveling problem with bltouch 3.1 on Anycubic i3 Mega S

Hi,

I upgraded my printer, an Anycubic I3 Mega S by adding the original bltouch v3.1.
I use the 2 endstop Z and not the bltouch for the z-home.

At each firmware upload I execute:

• loading default values
• M500
• turn the printer off and on again

This is what I did:

1. I load the Firmware, configured with the attached files, with the z-offset at 0.
2. I calculate the Z offeset with one of the many procedures that are around and I get Z-2.06
3. I reconfigure the Firmware with this Z-offset value and reload on the printer
4. command G28 followed by G29 (25 points)
5. M500 to save the leveling

When starting a print, I run:

• G28 W
• M501
• M420 S1 Z3

So without turning off or doing anything else I perform a simple print and the nozzle scratches the surface, it goes too close to the surface.
If instead, just after leveling and the following M500, I turn the printer off and on again, the first print does it right, perfect.
The next print is still too close to the platen, even if it doesn't scratch the platen as it did at the beginning, it just prints badly.
I've run some tests, and the only way to print correctly seems to be to run:

• G28
• G29
• M500
• Turn off printer
• Turn on printer
• start printing

Changes in the configuration file to enable bltouch:
File Configuration.h

1. uncomment I3MEGA_HAS_BLTOUCH so use AUTO_BED_LEVELING_BILINEAR
2. set Z_MIN_ENDSTOP_INVERTING to true
3. set NOZZLE_TO_PROBE_OFFSET to {-2, -23.5, -2.06}
4. set PROBING_MARGIN to 35
5. set Z_PROBE_FEEDRATE_FAST to 600
6. set Z_PROBE_OFFSET_RANGE_MIN to -50
7. set Z_PROBE_OFFSET_RANGE_MAX to 50
8. comment MIN_SOFTWARE_ENDSTOP_Z
9. uncomment RESTORE_LEVELING_AFTER_G28
10. set DEFAULT_LEVELING_FADE_HEIGHT to 3

File Configuration_adv.h

1. uncomment BLTOUCH_HS_MODE and set it to true
2. uncomment BABYSTEPPING
3. uncomment BABYSTEP_ZPROBE_OFFSET

Now I ask:
am I wrong something in the configuration?
can it be a firmware bug?
is another problem only with my printer?

Could you give me some indications? what to change and / or try?

Thanks!