Don't think it is a bug, Netflix does the same thing.
As far as I know it is the app that is coded to takeover music/sound playback on the phone when it is opened even though the apps don't actually play anything.
Apple should definitively do something about this but it is also the developer of these apps that are screwing us over.
I haven't really checked recently if it still behaves like following but atleast it used to.
When you start a new message conversation it will use the default number set in settings for imessage and this setting has to be set on every device.
When you reply to a message it will use the number or email that the last message was sent from or to on all devices as long as the number or email is activated for imessage in settings on the device (which it should be by default).