I manually updated to 9.0 by copying the image file LibreELEC-Odroid_C2.arm-9.0.0.img.gz to /storage/.update and rebooting, as per Update LibreELEC [LibreELEC.wiki]. With 9.0.0 my odroid-c2 is not powered on via CEC when the TV is powered on. I had to unplug the odroid's power supply and plug it back in to get the odroid to boot up.
I downgraded to 8.2.5 by copying LibreELEC-Odroid_C2.aarch64-8.2.5.img.gz to /storage/.update and creating /storage/.update/.nocompat. I then shutdown the odroid, unplugged the power supply, and plugged it back in. LibreELEC was downgraded to 8.2.5. I shut down the odroid which turned the TV off, as it should have. When I turned the TV back on the odroid powered up.
Edit: The same thing was reported at Odroid_C2 8.90.006 no CEC Power on by connected TV - LibreELEC Forum with a beta version.