Lockscreen rotation + fingerprint reader = bug [edited]
 : I found the reason of the issue described below, in order to trigger tabs in Chrome I'm used to change DPI value manually... Once changed back to standard DPI values, the trouble does not occur. This message can be deleted, thanks.
I have noticed that if screen rotation is enabled, the lockscreen will 180° rotate too.
Now the trouble is with the portion of the screen that gets illuminated for the fingerprint scanner, as it is not anymore on the appropriate spot.
In such conditions, the fingerprint cannot be read.