This happened to me as well and to say that I am a little upset and disappointed about this would be an understatement! I have 3 DS accounts on 3 different devices and rec’d this gift on one of my accounts on one device but I was away for 2 weeks and when I got home and played DS on my other 2 devices this gift was not in my storage and I had nothing in my goals about it either! I’ve also recently found out that when a neighbor gifts you gold that gift/gold disappears from your news feed if you don’t accept it within a certain amount of time also! This really makes no sense to me as other old neighbor and/or map requests from your neighbors will still be there just not the gifted gold!!
Why would they even think to make those things expire?!? Maybe for the same reason there is an option to refuse gold from someone. Like, who refuses gold?