I’ve had this before – use a different HDMI cable.

My extension is 1.3a spec rated (printed) and my HDMI cable didn’t specify anything (an old one). The signal would cut out at random to my monitor and refused to be seen via my HDMI over Ethernet extender to the TV.
I swapped it for a 1.5 metre 1.3 spec rated cable from Amazon and I have no issues now.
In short, I would make sure that the generations of HDMI cable you are using all match – no need to spend ££’s on cables though.