So just added a 4th ST button yesterday and it too is working great..no issues with on/off or anything. Just wanted to report my experience. I do not have as many Zigbee devices as you so likely that is the main issue.
This should be possible to narrow down by looking at the live or past logs for your button device (if it's not too late), or the "Events" history page on the device page itself in the admin UI. You'll know it's a problem with the button if you see two events in quick succession for only one push.
