Why are my objects changing unexpectedly?

Why are my objects changing unexpectedly?

I've got a very basic example where I'm reading some data into a class
from JSON, and my objects are getting corrupted somehow. I suspect I'm
missing some detail about how properties/ARC is working, but I'm not sure
what it is, or how I'd track it down.
I've reduced my original code so that the problem is obvious. However this
means that its not clear why I'm using custom properties etc. - my real
code has more functionality in there...
The issue can be seen in the last line of the Test.m file. The first
constructed object now contains data from the second object rather than
the value it originally contained.
Any advice on what I'm doing wrong and/or how to track down this kind of
issue would be greatly appreciated.
ANBNote.h
@interface ANBNote : NSObject
@property (nonatomic,readwrite) NSArray* references;
- (id)initWithJson:(NSDictionary*)data;
@end
ANBNote.m
#import "ANBNote.h"
@implementation ANBNote
NSArray * _references;
-(id) init {
if(!(self=[super init])) return nil;
_references=@[];
return self;
}
-(id)initWithJson:(NSDictionary *)jsonObject {
if(!(self = [self init] ) ) { return nil; }
_references = jsonObject[@"references"];
return self;
}
-(void) setReferences:(NSArray *)references {
_references = references;
}
-(NSArray *)references {
return _references;
}
@end
Test.m
...
NSDictionary * d1 = @{@"references":@[@"r1",@"r2"]};
NSDictionary * d2 = @{@"references":@[@"q1",@"q2"]};
ANBNote * n1 = [[ANBNote alloc] initWithJson:d1];
NSLog(@"R1 = %p, %@", n1.references, n1.references); // Prints r1, r2 - as
expected
ANBNote * n2 = [[ANBNote alloc] initWithJson:d2];
NSLog(@"R2 = %p, %@", n2.references, n2.references); // Prints q1, q2 - as
expected
NSLog(@"R1 = %p, %@", n1.references, n1.references); // Prints q1, q2 - Oh
No!
Note that if I remove custom references property functions and rely on the
compiler generated version everything seems to behave correctly.