I am having the same or similar problems. Massive HDMI problems: TV LG 55UH950V, Raspberry 4B 4GB, LibreELEC-RPi4.arm-9.1.501.img.
CEC works only sometimes and sporatic on one of the three TV HDMI inputs. I couldn't find any pattern yet.
Further: HDMI on TV is only active if TV is already running with RP HDMI selected and RP is switched on after. If RP is already running and TV is switched on later a black picture appears only.