I had some communication with the Blender and ffmpeg developers about issues with transparency
For example, the preview panel in Blender was corrupted and some input images or videos were corrupted in OBS
The root cause was in ffmpeg, it is
fixed here. The
issue is also tracked in Blender.
I produced an updated version of my Debian backport of ffmpeg 4.3.1, it now includes both the POWER9 optimizations and the fix for this particular issue. The version with the fix is ffmpeg_4.3.1-5~bpo10+2
If you already used packages from apt.debify.org then you can simply do
apt install -t debify-buster-backports ffmpeg
If you did not already use it, you need to add the apt source before installing/upgrading your ffmpeg:
$ wget -O - http://apt.debify.org/add-apt-debify | bash
apt install -t debify-buster-backports ffmpeg