FFMPEG: video encoded with vp9/yuva420p but loose transparency?

I’ve an issue on ffmpeg usage. Guess it’s my fault, but I don’t understand why. I tried to study the documentation and the mass of google stuff regarding ffmpeg, but I did not find any hints for it which would help.

The issue is following.

I’m using ffmpeg with a pipe input as rawvideo (ARGB 8x8x8x8 pixel data). In general, it works and produces a correct video like a charm.
But now I’m stuck to get video with transparency, so I’ve changed to webm which supports transparency.

As I can see on the log it uses rawvideo ARGB input as expected and do the export as vp9/yuva420p which should provide alpha support also as expected.

ffmpeg.exe -y -use_wallclock_as_timestamps 1 -f rawvideo -pix_fmt argb -r 60.0 -s 600x600 -i - -q:v 0 video.webm

ffmpeg version 5.0.1-full_build-www.gyan.dev Copyright (c) 2000-2022 the FFmpeg developers
...
Input #0, rawvideo, from 'pipe:':
  Duration: N/A, start: 1655899073.916667, bitrate: 691200 kb/s
  Stream #0:0: Video: rawvideo (ARGB / 0x42475241), argb, 600x600, 691200 kb/s, 60 tbr, 60 tbn
Stream mapping:
  Stream #0:0 -> #0:0 (rawvideo (native) -> vp9 (libvpx-vp9))
[libvpx-vp9 @ 0000027775a49f80] v1.11.0-133-ga3afed22a
[libvpx-vp9 @ 0000027775a49f80] Neither bitrate nor constrained quality specified, using default CRF of 32
Output #0, webm, to 'video.webm':
  Metadata:
    encoder         : Lavf59.16.100
  Stream #0:0: Video: vp9, yuva420p(tv, progressive), 600x600, q=2-31, 60 fps, 1k tbn
    Metadata:
      encoder         : Lavc59.18.100 libvpx-vp9
...
frame= ...

But if I view the video it has no alpha.

So, then I checked the video with ffprobe and to my surprise it now tells me that the stream is encoded by yuv420p which not providing alpha support!

ffprobe.exe -i video.webm
ffprobe version 5.0.1-full_build-www.gyan.dev Copyright (c) 2007-2022 the FFmpeg developers
...
Input #0, matroska,webm, from 'video.webm':
  Metadata:
    ENCODER         : Lavf59.16.100
  Duration: N/A, start: 0.000000, bitrate: N/A
  Stream #0:0: Video: vp9 (Profile 0), yuv420p(tv, progressive), 600x600, SAR 1:1 DAR 1:1, 60 fps, 60 tbr, 1k tbn
    Metadata:
      alpha_mode      : 1
      ENCODER         : Lavc59.18.100 libvpx-vp9
  

I’ve also tried many this and that options from several google research but non of them leads to the goal.

Any hints are welcome to get the video encoded with transparency.

PS: I also checked that the input pixels used for pipe input are containing correct alpha information.