Those who have an issue with the double tap on scanner, or want a double tap on screen to lock, the best option seems to be the Microsoft Launcher.
It has a whole lot of customisations to lock the screen, including double tap, swipe, etc.
I was surprised when I saw it and it seems to work very well.
The built in double tap on scanner works well if you tap gently with the bottom half of the scanned finger. It works well, but is horribly inconsistent. The Microsoft Launcher should work as a good substitute till Smartron irons out the bug.