Oh, that is a pain, maybe the avr isn't fully compatible with whatever signal Panasonic implemented for control, as nothing in the earlier versions ever seemed to be fully standard? Or it just doesn't support the wakeup control signals full stop.
Sorry I can't offer any more advice that, I haven't had much experience of any other TV equipment in the better part of a decade apart from trouble shooting family stuff and drooling over things I can't afford.
Edit: What avr is it?
I actually managed to finally get this working.
Went through a lot Of settings a lot of hassle.
I think it was the TV that's an issue.
I had to
Enable vierra link on TV (not called hdmi control. Or CEC)
I had to enable hdmi control on amp.
It didn't stop there. Turning on the amp (set to chromecast) didn't turn the TV on.
Something has to stream (say netflix) for. It to activate.
Still not the end.
For some reason this kicked the TV into DVB (freeview) mode. This is bizarre default behaviour, a hdmi source has turned you on.. Default to freeview. I have no idea why. I even set the default to 'hdmi 1'.
I tried different hdmi ports. No luck.
I tried setting default hdmi on TV. No luck.
What may have been the issue (I'm not sure, I was flicking so many settings) is changing a setting on a button called 'vierra link' from TV to 'home cinema'.
There was no explanation of what this even was.
Main issue is the tvs menu system is ****. Poor explanations. Menus hidden in weird places. To get default hdmi I had to enable 'hotel mode' by holding - channel and pressing AV 3 times... Wtf.
Anyway after much pain, I now just have to 'turn on the amp' to my home assistant.
After this saying 'play X on living room TV' kicks the rest off.
Thank you all for your help. This is one I wouldn't have figured out. Never knew amp could control the TV like that.
Got to properly try it tonight as having issues for sound from TV speakers rather than the 5.1.