I have a Beebox and set it to fast boot in the bios. Then i had the same problem. Have you checked that fast boot is disabled?
Fast boot is disabled.
I am pretty sure I tried all combinations and finally found one that worked. I always did a reset of the CMOS and all HDMI ports on all devices between all my tests. I now have not seen a black screen the whole day and do not need a delay.
I have the receiver connected to HDMI1 of the Beebox and the receiver connected to HDMI2/ARC on the Samsung TV. I use the edid string I collected while it was directly connected to the TV. I do not need a delay anymore.
As soon as I use HDMI3 on the Beebox, things become unpredictable. After one to ten restarts or normal startups it will give the black screen. If I connect the TV to the receiver and then to HDMI3 on the Beebox, it works as long as I use a delay?
The HDMI handshaking on the Beebox is pretty unreliable in my opinion.
Another thing (Nothing to do with the black screens), I had to add <minimumsamplerate>48000</minimumsamplerate> to the audio section of the advancedsettings, because suddenly my receiver is unable to play audio with a lower sample rate. If I use my old Asrock ION 3D with Libreelec connected to the same receiver, it will play them?