Image sticking Issue when using iOS replaykit Broadcast Upload Extension - replaykit

Question

I am testing Broadcast Upload Extension under iOS11.4.1 or 12.0 with iPad6.
After extracting the YUV data from CMSampleBufferRef, and saving those data to a file, I get some bad images. The issue seems like that the last frame image still remains on the new frame, and not refreshed. Maybe the data is not ready, when coming out from callback.I wonders how to avoid this issue.
one bad image here
another bad image
- (void)processSampleBuffer:(CMSampleBufferRef)sampleBuffer withType:(RPSampleBufferType)sampleBufferType {
switch (sampleBufferType) {
case RPSampleBufferTypeVideo:
{
CFRetain(sampleBuffer);
size_t bytes = 0;
char* data = NULL;
size_t bufwidth, bufheight, bufstride;
CVPixelBufferRef pixelbuf = CMSampleBufferGetImageBuffer(sampleBuffer);
CVReturn cr = CVPixelBufferLockBaseAddress(pixelbuf, kCVPixelBufferLock_ReadOnly);
for (size_t i = 0; i < CVPixelBufferGetPlaneCount(pixelbuf); i++)
{
bufwidth = CVPixelBufferGetWidthOfPlane(pixelbuf, i);
bufheight = CVPixelBufferGetHeightOfPlane(pixelbuf, i);
bufstride = CVPixelBufferGetBytesPerRowOfPlane(pixelbuf,i);
data = (char*)CVPixelBufferGetBaseAddressOfPlane(pixelbuf,i);
if(bufwidth == bufstride)
{
size_t ylen = bufwidth*bufheight;
fwrite(data, ylen, 1, _file_yuv);
}
else
{
size_t factor = bufstride/bufwidth;
bytes = bufwidth * factor;
for (j = 0; j < bufheight; j++)
{
fwrite(data, bytes, 1, _file_yuv);
data += bufstride;
}
}
}
CVPixelBufferUnlockBaseAddress(pixelbuf, kCVPixelBufferLock_ReadOnly);
CFRelease(sampleBuffer);
}
break;
case RPSampleBufferTypeAudioApp:
break;
default:
break;
}
}