objective-c - 使用 Unicode SMP 条目创建 NSCharacterSet 并测试成员资格——这是一个错误吗？

Question

给定代码：

NSString *a    = @"a";
NSString *clef = @"";
UTF32Char utf32char = 0x1D11E;  // 

NSCharacterSet *cs1 = [NSCharacterSet characterSetWithCharactersInString:@""];
NSCharacterSet *cs2 = [NSCharacterSet characterSetWithCharactersInString:@"a"];
NSCharacterSet *cs3 = [NSCharacterSet characterSetWithCharactersInString:@"a"];
NSMutableCharacterSet *mcs1 = [NSMutableCharacterSet characterSetWithCharactersInString:@""];
NSMutableCharacterSet *mcs2 = [NSMutableCharacterSet characterSetWithCharactersInString:@""];

[mcs1 addCharactersInString:clef];
[mcs1 addCharactersInString:a];

[mcs2 addCharactersInString:a];
[mcs2 addCharactersInString:clef];

NSLog(@"cs1 - %@",  [cs1  longCharacterIsMember:utf32char] ? @"YES" : @"NO");
NSLog(@"cs2 - %@",  [cs2  longCharacterIsMember:utf32char] ? @"YES" : @"NO");
NSLog(@"cs3 - %@",  [cs3  longCharacterIsMember:utf32char] ? @"YES" : @"NO");

NSLog(@"mcs1 - %@", [mcs1 longCharacterIsMember:utf32char] ? @"YES" : @"NO");
NSLog(@"mcs2 - %@", [mcs2 longCharacterIsMember:utf32char] ? @"YES" : @"NO");

我得到以下输出：

cs1 - YES
cs2 - NO
cs3 - NO
mcs1 - YES
mcs2 - NO

1. 为什么只有 cs1 似乎可以正常工作（对于不可变字符集）？
2. 为什么顺序对于可变字符集很重要？

这是一个错误吗？ObjC 的内部 UTF-16 表示存在一个已知问题（现在仍然如此吗？）？