Does the display issue only happen on a specific channel/content or across other channels installed on your Roku device? We would recommend changing the picture setting during playback by following the steps below:
After playback begins, press the Star button on your Roku remote to open the Options menu.
Scroll up or down to Picture mode.
Change the picture mode using the left and right buttons on the directional pad. The directional pad is plus-shaped and is in the center of the remote.