Leica monochrom cameras are now on their 3rd iteration.
The results have aged well: not not only are they as valid as ever, but the damage done to image sharpness by using rangefinder focusing is far more drastic on a 40MP sensor. Because the rangefinder does not let you see the changing in focus that occurs with a color filter; the rangefinder is its own optical system.
Focus using magnified Live View when a color filter is mounted or you will get soft images guaranteed (unless your own error is in the inverse of the focus shift caused by the filter, by pure luck). This fact is trivial to demonstrate even without recording an image: focus in visible light in magnified Live View, then screw on a red filter and watch the image go very soft (out of focus).
In truth, the accuracy of the rangefinder focusing is is inadequate on a 24MP sensor, hopeless on a 40MP camera*, and it’s almost impossible to adjust the camera against more than 2 or 3 lenses and this can take months even if you can get Leica to do it for you. Use the EVF and forget about rangefinder focusing.
* Rangefinder focusing on a 40MP camera is a delusional design for gullible customers. Even Leica implicitly acknowledges this by stating that no additional accuracy is possible with the 40MP sensor vs the 24MP sensor.