AVAudioRecorder and iOS 5.1

I have an application that records audio using AVAudioRecorder. It works fine on devices with iOS versions prior to iOS 5.1, but with the latest version, it does not increase the file size of the audio recording, which remains at a constant 4k speed.

// init audio recorder here so recording could be started immediately            
NSDictionary *recordSettings = [NSDictionary 
                                dictionaryWithObjectsAndKeys:
                                [NSNumber numberWithInt:kAudioFormatAppleIMA4],
                                AVFormatIDKey,
                                [NSNumber numberWithInt: 1],
                                AVNumberOfChannelsKey,
                                [NSNumber numberWithFloat:44100.0],
                                AVSampleRateKey,
                                nil];           

// Get temp path name
NSArray *paths = NSSearchPathForDirectoriesInDomains(NSCachesDirectory, NSUserDomainMask, YES);
NSString *cachePath = [paths objectAtIndex:0];
BOOL isDir = NO;
NSError *error = nil;
if (![[NSFileManager defaultManager] fileExistsAtPath:cachePath isDirectory:&isDir] && isDir == NO) {
    [[NSFileManager defaultManager] createDirectoryAtPath:cachePath withIntermediateDirectories:NO attributes:nil error:&error];
}
self.audioFilePath = [cachePath stringByAppendingPathComponent:@"audio.ima4"];
NSURL *audioFileURL = [NSURL fileURLWithPath:self.audioFilePath];
AVAudioRecorder *anAudioRecorder = [[AVAudioRecorder alloc] initWithURL:audioFileURL 
                                                               settings:recordSettings
                                                                  error:&error];
self.audioRecorder = anAudioRecorder;
[anAudioRecorder release];
 [self.audioRecorder prepareToRecord];

      

        
        
        
      

    

(wait a while, user clicks record) [self.audioRecorder record]; (wait a while, 5-10 seconds, user stops stop) [self.audioRecorder stop];

I posted debug statements that check the temp file size and is 4096 bytes throughout the entire write, and only 4130 bytes after the stop call.

I don't see any way around this. Is this a bug in iOS 5.1?