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.
https://lists.freedesktop.org/archives/wayland-devel/2016-April/027822.html
commit 886b5a2cd8c785e48ddf5ceb4a902cd1f518d2a9 Author: Peter Hutterer <peter.hutterer@who-t.net> Date: Mon Apr 4 10:06:36 2016 +1000 touchpad: add a middle button software area
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