I am having the same issue. I have tried the items you suggested. I have done a factory reset. I have plugged directly into the wall outlet instead of a plug strip. I have gone back to original remote that came with the Onn Roku Tv, instead of the additional remote we purchased with voice commands. I have made sure the screen savers are disabled, turned off any power saving options, confirmed there aren't any CEC connections enabled, confirmed all software is up to date, and did a factory reset. The TV still turns itsef back on.
