In his write up of the Leica Q2, Jonathan Slack describes how the small “zoom/lock” button on the back of the camera (near to the thumb indent) can be assigned the following functions: Digital Zoom, AEL / AFL, AEL or AFL. He then goes on to describe how when the the AFL function is chosen, in order to use Back Button Focus (BBF), focus is disabled from the shutter release only while the lock button is pressed:
if you choose AFL for the zoom lock button then focusing is disabled from the shutter release (as long as you keep pressing the zoom lock).
If you press the AFL button to focus (or AEL button to lock exposure) you must keep it held down until you have pressed the shutter.
This is not how users of BBF expect it to behave. If the Q2 does work this way, it means it would be impossible to lock focus and recompose without focus changing.
Update Wed 20th March 2019
Today I went into my local Leica dealer and tried out the Q2, and sad to report that BBF works in the broken way described here, and further its looks like Exposure Compensation in Manual mode with Auto-ISO is also not working, which does work on the Q
In the previous Q model, once BBF is setup, it acts as expected – that is: press and hold to continuously focus, and then release to lock focus, which will not change when the shutter is pressed even if the frame is recomposed. This allows you to switch between continuous and single focus without changing any settings and to easily lock focus.