The client is just terrible.
Me and my friends did a load of mystery gifting over Christmas to each other and have played 3-4 times a week since then. Everytime I login I still get a message saying they have opened their gifts (that I sent 6 months ago).
Fun fact about this: It is because clicking the X dismisses the notification without acknowledging it. Not a bug, but not the best possible design.
Extra fun fact: I just filed something to address this. Should be fixed soon.