SOLVED !!!
SOLVED !!!
The problem was caused by the tongue of the microswitch being bent downward. So the switch was not being activated properly by the limit switch block.
The switch did not seem to activate even when pressing the tab with fingers.
I solved it by bending up the tab.
I would say that this method of handling Z-axis homing is not very reliable because the tongue of this microswitch is made of a very light sheet metal and when the board is lowered it is probably always subjected to pressure that inevitably deforms it over time. This requires the repetition of the bed leveling calibration procedure, which, however, becomes evident only after the hot ends have scratched the plate.
There are two cases, either replace the microswitch with one of the same size but with a stronger tongue, or replace the limit switch mechanism with the same optical system used on the X-axis.
The designers’ choice seems to me to be very questionable because while the X-axis has no major zero promblims (if the print starts a few thousandths of a millimeter further to the left or right, it changes nothing), for the Z-axis, on the other hand, repeatability is essential.