Ive always had this problem even in version 8 of LibreELEC
Wake up some mornings and my TV would be on, on the HDMI port the pi is connected to with LibreELEC.
I solved it by changing a setting on my TV.
I have a lg TV and when I enable hdmi CEC it automatically enables a setting called auto power sync.
Disabling this feature stopped the pi from waking my TV from standby permanently.
It's never done it since