Also enabling LED patterns finally allowed me to confirm what I was suspecting. With screen turned off if you put your finger on the sensor it will only turn the screen on if it recognises the fingerprint. And even though it recognised the fingerprint it will not unlock the phone, you still need to put the finger on the sensor again. This is kind of stupid, because often it won’t recognise it for the second time, even though it did just 2 seconds before.
So shouldn’t it unlock the screen immediately after it recognises a valid fingerprint?