Video not loading on Apple Devices

[baltermia]

I'm generating a video file (with no sound) out of multiple GIFs. The GIFs are loaded from files but the final video is sent somewhere else using a MemoryStream. The ffmpeg saving logic looks like this:

IEnumerable <IVideoFrame> frames = CreateFrames();

MemoryStream outStream = new();

RawVideoPipeSource video = new(frames)
{
    FrameRate = _options.FPS,
};

await FFMpegArguments
    .FromPipeInput(video, options => options
        .WithFramerate(_options.FPS))
    .OutputToPipe(new StreamPipeSink(outStream), options => options
        .WithVideoCodec("vp9")
        .ForceFormat("webm")
        .UsingMultithreading(true))
    .ProcessAsynchronously();

The GIFs are loaded into memory using SkiaSharp and the frames are also generated using that library.

Now, on Windows (when I save the MemoryStream to a file using FileStream), I can view the video. Even when sending the stream over to a messenger API, it's viewable on Android devices.
But for Apple Devices nothing happens. I was tested on my iPhone and MacBook.

On Windows, it's also not perfectly working. For example, the video has no length for some reason. So I can only start/stop and go to start. And for some other people to whom I sent the video (over the messenger), they told me the video was 420 hours long.

I tried using different codecs and formats but that either tells me that the arguments are invalid or that the pipe is broken.

Any help would be greatly appreciated.