There's just as much possibility it's the NUC as it is the adapter.
Obviously I've asked the product vendor to investigate but I suspect they will give the same comment as you and blame the NUC.
I don't understand what you mean by testing another adapter, are you recommending I buy another adapter and try it.
AFAICS there aren't any other external CEC adapters that suit my needs.
But since you have replied with the expected response perhaps you can help by providing information on BIOS settings that may help.
I can't remember now but does the NUC BIOS provide the ability to specify a custom EDID and if so will that cause the HDMI video signal to be provided regardless of what is or is not connected?
Ian