I don't know if this is a known bug, I have trouble exporting some project: after the resulting movie was encoded, it turns out the movile file that was being generated was only 44 bytes long. I tried exporting to YouTube, to the Videos app, to the Files, and everytime the generated movie was only 44 bytes.
After changing various settings, I found out a way to have LumaFusion export the movie: it turned out that, when I selected a quality that generated a file that would be more that 2GB, the rendering would silently fail, and the movie would be only 44 bytes. I changed the quality setting to a lower value, the file was smaller, and the rendering proceeded just fine.
I am not 100% sure if the bug is being triggered by a high quality, or a very big output file, though I have been able to output high-quality movies for some projects that were not as long, and without any issues, which make me believe that the bug is probably related to something not being able to render file bigger that some size (2GB or 4GB, not sure).
Let me know if there is anything I can do to help you pinpoint the issue!