The power-savings feature introduced by Roku OS 11.5 to some Roku devices may cause some some unwanted HDMI related issues.
First thing, just press the Home button on the Roku Remote to wake up the Ultra LT and see if that resolves it.
If not, then try disabling the Power Saving feature. Settings/System/Power/Auto-Power Savings/20-mins [Disable]
Just for clarity, there is no powering down the Roku device. You press the Home button when you are finished streaming and the Roku device should enter a sleep mode after some time of inactivity. You then press the Home button when you want to wake the Roku and begin streaming again.
If you are just pressing the Power button on the Roku remote, you are just powering down the TV. (Power and volume buttons are TV controls and do not affect the Roku device). If you are doing this, then the Roku continues to stream in background until the Power Savings feature (if enabled) is kicking in. When this occurs, some have reported difficulty with either video or audio signals lost and regaining connection, requiring a System Restart/reboot of the Roku device to re-establish the HDMI handshake.
Roku Community Streaming Expert
Just another Roku user... I am not a Roku employee.
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