Given the details you provided, this is most likely an issue with the HDMI cable, the seating of the cable, or may relate to a specific channel or content that you are watching.
I would recommend replacing the HDMI cable connecting your device to your TV, ensuring that you are using a HDCP 2.2 certified, high-speed HDMI cable, being cautious not to bend the cable or damage the connectors, and try plugging the device into a different HDMI port on your TV.
If you still observe the issue occurring from there, please take note of exactly which streaming service/channel you were watching, along the specific content title (movie/tv show). After you see the issue on screen, generate an issue report tracker ID by pressing the Home button 5 times quickly, followed by the back button 5 times quickly. Please send me a PM with the details above, along with your Roku account email address, and the Roku device serial number from Settings>System>About once more.
I'll help take a closer look and share your report with our team for further investigation.
Roku Community Manager