Hi @MisterSnuffles
Welcome to the Sonos Community!
Do you see the stated volume level in the Sonos app change? If not, then I suspect the TV is the cause.
Please try unplugging your TV from power for at least 2 minutes, then test again.
If the issue persists, I can only recommend you get in touch with our technical support team, who have tools at their disposal that will allow them to give you advice specific to your Sonos system and what it reports.
I hope this helps.
"Common sense is the collection of prejudices acquired by age eighteen." - Albert Einstein