AFAIK it is always important to have the HDMI device(s) active when Kodi starts otherwise several (if not all) things related to HDMI port(s) won't work. The reason is that Kodi performs an autodetect at start (only).
In general, systemctl restart kodi should help without complete reboot.
I think that having the TV (And soundbar) both on did it. I've restarted the TV a few times, and Kodi is remote-controllable.
I'm testing the Matrix alpha release with a Raspberry Pi 4 Model B Rev 1.1 (nightly-02021031-73c0ce6).
Now that HD Audio is supported, I connect the RPi4 directly to my Soundbar (SB36514-G6) which is connected to my TV (Sony X850G) via eARC.
If I boot (or reboot) my RPi, the CEC remote works perfectly.
If I turn the TV off, and then on again, it doesn't work anymore.
The cec-client reports that it's "unable to open the device on port Linux" even when the remote control is able to communicate with Kodi.
