With previous versions of HDMI to use ARC, CEC needs to be on. ARC will not work if CEC is off. CEC can also cause problems with turning on/off equipment and switching inputs. The more devices you have connected the more likely it will mess something up.

The new version of HDMI (2.1) will allow eARC to work with CEC turned off to solve some of the input switching and power problems. However, not all devices have that option in their menus to turn off CEC but leave eARC on. It is up to the manufacturer/device whether they require CEC to be on for eARC to work.

For eARC to work all devices need to be capable of eARC and that includes the HDMI extender. That's why if a device has the option to turn of eARC but still use regular ARC with CEC turned on, it may fix your problems with the extender.

HDMI is a messed-up standard!