Out of 10 clicks with both fingers in the software button areas, I get 5-7 as middle-clicks at best, the rest is either left or right buttons. event-debug --verbose shows that the button state machine detects the right button areas but does not send the right button event then.
tested on a T450 and a T440 and it's pretty much the same effect on both. In most cases this isn't a libinput bug (all cases?) but the type of interaction we do with the middle button.
Primarily it's used for paste which is a single click immediately after putting the fingers down. It's very common to trigger the click before the sensor detected the second finger, resulting in a left or right click only. I don't think we can meaningfully put delays in without making actual button clicks sluggish, so I think that changing the software button code to include an actual middle button area is the only/best solution.
Author: Peter Hutterer <firstname.lastname@example.org>
Date: Mon Apr 4 10:06:36 2016 +1000
touchpad: add a middle button software area