I'd like to confirm, this issue.
I tried a couple of times
- on two different raspberry pi 4s (one with 1GB RAM, the other with 2GB)
- with the latest "stable" LE 10.0.2
- with exactly the same hdmi-cables
- using different SD-Cards (since boot from USB-stick stopped working as well)
- original pi4-power-elements
... each time already during fresh install (right after the rainbow-square), LE loses the HDMI-signal.
I can hear the sound of the Interface, but the screen (TV and Monitor) just stays black.
I have to turn off/on the Monitor OR switch the source on the TV, to get the LE-Screen.
The after first install and reboot ... again: screen stays black.
What can I do to get 10.x to find the connected screen (tv or monitor, borh 1080p) after reboot?
When I downgrade to LibreELEC-RPi4.arm-9.2.7 the same setup, cables, Monitor, TV ... all work, no black screen. After reboot, the pi's find the Monitor or TV instantly. As they should.
It‘s not the cables. No fancy raspi-case being used, no special Hardware (just rpi4s)
I used to install LE in a couple of minutes, now I fumble around forever, and in the end it just doesn't work ... 
Looks like it's not just me ... https://www.google.com/search?client=…+on+RPI+4+to+TV
LE used to be super stable, easy to use for my needs (as PLEX-Player), but now it seems like I can't update to 10.x ... too bad 
I use this for my grandparents, and can't make them start a shell to debug issues or change their display configs ...
