You are too faster for me, sorry.
My first (fast) test don't use Nested EDL. Tests did with
"cinelerra-5.1-ub16.04-20201031.x86_64-static" and
"CinGG-20221031-x86_64-older_distros-multibit.AppImage" on
UbuntuStudio_16.04, works good for PNG with alpha channel.
I followed your steps.

You can download my project for test to
https://transfer.sh/x12pY1/test-alphaChannelRender.tar.gz
In the compressed file you can find the project file (.xml) and its
files for test.
"test-render-1.png" is the render file;
Render-->
- File Format: FFMPEG | png

---

- Video:
     - Compression: png.png
     - Pixels: rgba
- Render range: One Frame

It would be better to have a demo file by Goat user and more information
about, before further tests.

IgorBeg