When I am watching cable TV, I will get a ROKU logo that looks like an HDMI adapter with an X above it flash on the screen and then go away after about 10 secs.
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This sounds to me like something your TV is flashing on the screen.
What is your TV make and model? Maybe someone familiar with it will know how to deal with your issue.
By any chance, does this happen about 20 minutes after switching from Roku to cable? If so, perhaps your TV feels the need to tell you that a device has powered down/gone to sleep?