I suspect this is a TV issue, and has nothing to do with your Sonos, which is merely a client to the CEC system managed by your TV. You could test this by unplugging your Arc from the TV’s eARC port and then replicating your issue.

This not being a TV forum, I’m a little more hazy on a fix, though. The three things I’d suggest are an update of the software on the TV set, a power cycle of the TV set (pulling the wall plug for two minutes), and a call to the TV manufacturer’ CS group.

I can’t think of any reason why a client (speaker) like Sonos could be involved. 

I’m just speculating: Search down all of the device menus for a “power save” option and disable it. (SONOS does not offer such an option)

CEC (Consumer Electronics Control) over the HDMI connection promises to streamline user interaction with an A/V system. The goal is to allow the operator to simply drop a disc into a player or turn ON a source, such as the ROKU, and and other connected devices will power up and select inputs. Unfortunately, there are very loose standards about which commands and when they are sent, resulting in oddities such as you are experiencing. Manufacturers usually claim its the other guys fault when something unfortunate happens. Then one or more of the manufactures issues an update that may or may not improve the situation.

It’s possible that one of your devices delayed sending it’s “OFF” command and a later stimulus prompted an “oops I forgot --- here’s that command”. You can systematically disconnect each input device to see if the problem goes away, but this may not suggest a final solution. Another approach that many find is successful is to insert a CEC Blocker into an HDMI cable. Usually only one is needed, but you will need to experiment with where you insert it. The blocker will disable CEC commands to and from that device. The only real down side for the operator is that the device with the CEC blocker will not automatically turn ON or encourage the other devices to power ON/OFF or select inputs.