We have custom video composition that alters frames and everything works fine except on some videos [AVAsynchronousVideoCompositionRequest sourceFrameByTrackID] returns nil CVPixelBuffer. It happens only at specific times of that videos, usually at start, all other processing goes well and sourceFrameByTrackID returns normal buffers with pixel data in it. At the same this videos are totally ok and plays well in Photos app (for example).What reasons can cause such strange behaviour?