A client setting Button1MotionMask only does not get motion events from emulated touch events.
To be more precise: TouchUpdate events that generate motion events will not be delivered because no TouchListener is set up if no client listens for button press events.
However, the emulated motion event before a TouchEnd will be delivered. UpdateDeviceState resets the event_filters to the normal motion mask before the emulated event is processed. EventIsDeliverable thus matches and the event is sent.
This is a mass change of bugs. Bugs assigned to me that haven't been updated in the last 3 years are closed as WONTFIX, because, well, let's at least be honest about it.
Please do not re-open unless you have a really good reason to do so (e.g. you're fixing it yourself). If it hasn't been fixed in the last 3 years, it probably won't be fixed anytime soon either. Sorry.