I have a similar issue, although the middle button does not stop working completely. What I notice is, that it takes various force to press the middle button to make it work over periods of times.
Sometimes it requires a really hard press. This happens for an unspecified period of time. Later, it requires a "normal" force to press it, which goes for a while as well.
I cannot compare it to other Z's. So I don't really know, what is the "Normal" force to press the middle button. However, compared to a Logi Mouse, it's been a forceful press from the beginning. It's even tiring during long sessions.