Xcode project works in Simulator, not on device - objective-c

Question

Still working on my flash card-themed app. Completely baffled. Lots of possibly salient info.
I have a version which works flawlessly, both in simulator and on multiple devices. But, I made a mistake when submitting to the App Store (wrong Bundle ID and Name), so I created a new project and copied and pasted the code and storyboard. (I didn't just move the files; I recreated them and copied the code.)
First problem was that it wouldn't build because "duplicate symbol _answers in" RootViewController.o and DescendantViewController.o. And there were 3 such duplicate symbols.
Yes, I have declared NSMutableArray *answers in the implementation of both my RootVC and DescedantVC, so I concede that's suspect, except.... it's the exact same code as in the version that works.
Nevertheless, I saw an opportunity to clean up my code, so I created a #property (weak, nonatomic) NSMutableArray *answers in my RootVC and changed the code in the root and descendant View Controllers to reference self.answers. It compiles without any problems.
It runs in the simulator perfectly. As I've just copied the entire code base to a new project, I test everything - no problems. I'm very excited, so I do all the steps to submit to the App Store, except the last one "Submit for Review."
I want to show off to my friend, so I attach my iPhone 6, compile it, run it and when I select "Play" - crash!
The following line is the culprit.
answers = [self pickWrongAnswersIgnoringCard:questionCard];
Both in the simulator and on the device, the last line of the this method is:
return wrongAnswers;
In both cases, wrongAnswers is an array with 3 items.
In the simulator, answers also has 3 items. On my iPhone, answers is nil.
Does anyone have any idea what this could be? The only thing I can think of is that I created the project that works over a year ago and the one that fails today, so maybe some of the Apple settings in the two projects are different? Otherwise, I'm stumped.
Edit
The function, as requested:
-(NSMutableArray *)pickWrongAnswersIgnoringCard:(FlashCard *)questionCard {
NSMutableArray *wrongAnswers = [[NSMutableArray alloc] init];
FlashCard *randomCard;
for (int i=1; i<4; i++) {
do {
randomCard = [self pickARandomCardFromDeck:wrongAnswersSource];
} while ([randomCard.name isEqualToString:questionCard.name] || [wrongAnswers containsObject:randomCard]);
[wrongAnswers addObject:randomCard];
}
return wrongAnswers; // device or simulator, wrongAnswers.count = 3
}

Answer 1

I figured it out, even if I don't fully understand.
I changed from #property (weak, nonatomic) to #property (strong, nonatomic).
That solved it for me.
Works on both simulator and device now.