In my experience I'd expect it to change ringtone volume by default (which is often linked to notification volume), unless you have a media app open or are in a call, in which case the button should control media or earpiece volume respectively. So I agree that this is unusual.
Unfortunately I don't know this device, so can't say whether it's a quirk of Meizu's software or something odd in your setup. I guess you could force stop any media players and see whether that makes a difference?
One my phone the volume pop-up also has a button to show all of the different volume settings, but even if yours has that too it's less convenient than it changing the one you expect directly (though it would provide a workaround).