I was about to create a thread that had the exact issue as OP. I have a rear entertainment monitor in a 2025 Sienna that would only display the black screen when trying to start up. The raspberry pi 5 is able to display EDID=ok without the sd card. Following OP's workaround of flashing to a previous libreelec 12.02 version solved the problem and it now plays video.
I also attempted several customizations in the config txt file, creating an EDID file as mentioned in the previous posts, and attempted forcing EGL settings. All resulted in the same issue of a black screen at start up. I also can confirm that my resolution is 720x576 even though my laptop does the full 1280x720 when connected to the Sienna.
Thank you EricD for the post! As I thought I was the only person with this unique problem. Just replying to bump up this issue for any others out there.