I had the same issue. TV was turning on after 10s I turned it to standby. All of above didn't help. It was the setting on TV that did the trick. Now I have Panasonic, so I guess every other vendor has different solution. In VIERA link settings there is one setting which enables turn off/on connected devices and that includes all which are on - HDMI as well. Turning that setting off, now TV stays off. 
yey - me 'appy
And I can still operate LEPi with remote from TV beside mouse/keyboard inputs
Pi 4 - 8GB, LEPi v9.2.6
I very much doubt it would be cable issue (unless you really have real cheap as paper one)
