While I cannot fully exclude HW malfunction as the primary cause, I still pretty much doubt it. And I have found a "workaround" for this weird bug. Well, workaround... Lets just say I discovered a bit less annoying way of unlocking the phone instead of typing my whole PIN.
When the phone is locked, I press the power button, which lights up the screen. From there I have the option to click on "emergency call". Then I place my thumb on the fingerprint reader and press "back" on the screen, as If I changed my mind and I don't need to make an emergency call. At that point the fingerprint read