png image stream synthesis webm video transparent channel lost

I have a png pipeline stream, similar to the one below

PipeInputStream pis = new PipeInputStream()
grabber = new FFmpegFrameGrabber(pis, 0);
grabber.setImageWidth(1920);
grabber.setImageHeight(1080);
grabber.setFrameRate(25);
grabber.setImageMode(FrameGrabber.ImageMode.RAW);
grabber.setFormat("png_pipe");
//Why `PixelFormat` is bgra, because I turn Mat
grabber.setPixelFormat(avutil.AV_PIX_FMT_BGRA);
grabber.start();

FFmpegFrameRecorder

recorder = new FFmpegFrameRecorder("output.webm", 1920, 1080);
recorder.setAudioCodec(avcodec.AV_CODEC_ID_OPUS);
recorder.setVideoCodec(avcodec.AV_CODEC_ID_VP9);
recorder.setFormat("webm");
recorder.start();
Frame frame = null;
while((frame = grabber.grabImage()) != null) {
    recorder.record(frame)
}
recorder.close();
grabber.close();

Description： Images saved as local png files before being written to the pipeline have a transparent channel, but the transparent channel in the generated webm video is lost.webm video generated directly from the FFmpeg command line also has transparent channels

FFmpeg command:

ffmpeg -i %6d.png -c:v libvpx-vp9 -y output.webm