I've never heard of that happening, but it must be something CEC related. In either your TV settings or the Roku settings, disable CEC. On the Roku, it's likely under System/Control Other Devices. Depending on the player, it might just say 1-touch play, or actually have a CEC function listed. Just turn it off.
Of course, it could be a defective remote for the TV, and whatever button you are pressing might be sending an input change signal instead of its intended purpose.
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Installed 3 Roku express 4K on 3 TV's that are Frontier Cable. When I change the HDMI 3 Roku to HDMI 1 cable Frontier, pushing any button on the Frontier cable remote defaults back to Roku. Happens on all three TV's.