Create an animated gif file from images in iOS

gistfile1.m

#import <UIKit/UIKit.h>
#import <ImageIO/ImageIO.h>
#import <MobileCoreServices/MobileCoreServices.h>

- (void)exportAnimatedGif
{
    UIImage *shacho = [UIImage imageNamed:@"shacho.png"];
    UIImage *bucho = [UIImage imageNamed:@"bucho.jpeg"];
    
    NSString *path = [[NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES) lastObject] stringByAppendingPathComponent:@"animated.gif"];
    CGImageDestinationRef destination = CGImageDestinationCreateWithURL((CFURLRef)[NSURL fileURLWithPath:path],
                                                                        kUTTypeGIF,
                                                                        2,
                                                                        NULL);
    
    NSDictionary *frameProperties = [NSDictionary dictionaryWithObject:[NSDictionary dictionaryWithObject:[NSNumber numberWithInt:2] forKey:(NSString *)kCGImagePropertyGIFDelayTime]
                                                                                                   forKey:(NSString *)kCGImagePropertyGIFDictionary];
    NSDictionary *gifProperties = [NSDictionary dictionaryWithObject:[NSDictionary dictionaryWithObject:[NSNumber numberWithInt:0] forKey:(NSString *)kCGImagePropertyGIFLoopCount]
                                                              forKey:(NSString *)kCGImagePropertyGIFDictionary];
    
    CGImageDestinationAddImage(destination, shacho.CGImage, (CFDictionaryRef)frameProperties);
    CGImageDestinationAddImage(destination, bucho.CGImage, (CFDictionaryRef)frameProperties);
    CGImageDestinationSetProperties(destination, (CFDictionaryRef)gifProperties);
    CGImageDestinationFinalize(destination);
    CFRelease(destination);
    NSLog(@"animated GIF file created at %@", path);
}

I have a problem with this piece of code, when I use this snippet to generate one gif with two static image, it works fine. But when I use ffmpeg to transform this gif to mp4, this mp4 file has a problem that one image is at beginning position and another is at the end. So when I loop this video, it acts strange.
this is my gif