Not sure if the Power button is doing this or your use of the remote (navigating) is waking up the Roku device attached to upstairs TV. Try disabling One-Touch Play on the upstairs TV and see if resolves your issue. (Settings..System..Control Other Devices..1-touch play).
Roku Community Streaming Expert Just another Roku user... I am not a Roku employee. Insignia RokuTV, Ultra 4660, Premiere+ 3921, Express 4k+ 3941, Streambar 9102