A TV that switches itself on at random may be taking orders from the Roku connected to it, and the cause is usually a setting rather than a fault.
A troubleshooting guide says that if a TV with a Roku attached keeps turning on unprompted, the Roku could be responsible, and the settings menus are the place to look. Two are worth checking first.
The main suspect is HDMI-CEC, short for Consumer Electronics Control. It is a long-standing part of the HDMI standard that lets connected devices send each other commands such as power on and switch input. Roku puts it under System settings as "Control other devices (CEC)," where 1-Touch Play lets the Roku wake the TV whenever the Roku becomes active. Switch that off and the Roku loses its permission to power the TV on. Menu names shift between models and software versions, so expect some hunting.
The second is Fast TV Start, a power option on Roku-built TVs that keeps the set in a standby state that wakes quickly. It trades a little extra standby power for speed. Turning it off is a sensible second test if CEC alone does not settle things.
The pattern is familiar. CEC is the feature that lets one remote run a whole stack of gear, and every TV maker ships its own branded version: Samsung calls it Anynet+, LG calls it Simplink, Sony calls it Bravia Sync. The same convenience that makes it useful also lets a streaming box, or a soundbar, power up the TV at an unexpected moment.
The practical approach is to change one setting at a time. That way you learn which one was responsible, and you know exactly which piece of one-remote convenience you gave up to get a quiet living room.