Is that not true. When I change media's volume I see a music icon and system sound for example notifications, sms or a call stay on same volume as in settings. , but when is that 2% I see an another icon and its change only system sounds.
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).
Yeh, Meizu uses a heavily customised and modified version of Android called Flyme OS. So could be doing its own thing. In fact Meizu designs it to look and work like Apple iOS. A China phone, usually distributed in Asia.