HDMI-CEC is a optional spec, so some equipment manufacturers don’t implement HDMI-CEC.
HDMI-CEC signals are sent to Pin 13 as you can see in the diagram above.
Plugging in non HDMI-CEC compliant device into your AV setup shouldn’t mess with your current HDMI-CEC setup but some equipment manufacturers who don’t adopt the optional HDMI-CEC spec sent garbage voltages through pin 13 which confuses the rest of your setup and stops HDMI-CEC from working on all your other equipment.
The quick and simple fix to this is to bend pin 13 on the HDMI cable which is going to be connected to the offending device in such a way that it does not make contact with the HDMI port of the said offending device. This is easier said than done but with a lot a patience and testing you will be able to fix this issue.
Another option is to buy HDMI-CEC less HDMI cables ie HDMI cables with Pin 13 absent or internally disconnected.I haven’t tried any of these cables but bending the pin 13 is effective as this.
Hope this helps if you any questions a drop a comment down below and I’ll get back to you.