Sorry for waking an old(ish) thread. The cable can definitely be the cause for CEC not working. Even a regular full size HDMI to micro HDMI (i.e. not using an adapter) can cause CEC to not work. I had three different micro to full size cables laying around and CEC only works with one of them.
I've been using one that didn't work, I have just thought that CEC was broken with LibreELEC and the Pi4 and haven't bothered to do proper troubleshooting, but now I can use my TV remote instead of a bluetooth keyboard or app!