Just installed Roku Ultra and everything works fine, however, when in Roku and we want to switch back to regular cable tv, we use the cable tv remote button to change the input from HDMI3 [Roku] to HDMI1 [Cable TV] and when doing so the power to the tv is turned off. When we power the tv back on, it comes up on cable tv [HDMI1]. The remote is setup to not power tv on/off so I don't know why it does. Any advice?
My first thought would be to try the TV remote instead and see if you get a different result.
Sometimes the HDMI CEC features may conflict between devices.
Try disabling the Roku Ultra's CEC One Touch Play feature.
(Settings/System/Control Other Devices/1-touch play (uncheck to disable).
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Otherwise may need to use the TV remote, enter the TV Settings, and look at other HDMI CEC settings. (Different TV manufacturers have different names for CEC).
https://hd-report.com/hdmi-cec-by-tv-manufacturer/