Messages ring tone no longer stops when I go to the message

I have one Message's sender ring tone (sent by our servers monitor in case of server trouble) set to a really annoying, very long ring tone in case something happens in the middle of the night, so I can be sure it wakes me up.

Up until recently, the instant I touched the lock screen, or went into Messages, the ring tone would blessedly cease.

But lately, even though I go to the message the ring tone keeps playing until it decides it is done.

Is there a way of fixing this behavior?

I've tried restarting the iPhone. (iPhone 6 plus, iOS 8.4)

Thanks,

doug