I have this same issue with my Roku Stick. The only thing I have found that fixes it is to reboot the Roku Stick by removing from the HDMI port and from power, wait about 10 seconds, and reconnect HDMI and power.
It got to the point I was having to reboot the Roku Stick two or three times in a two-hour movie.
I thought I had solved the problem by moving from one HDMI port to another. I thought it was a bad HDMI port. This was five or six months ago. Now, the same issue has followed the Roku Stick leading me to believe the Roku Stick is the culprit. Two HDMI ports on the same TV going bad on their own? I doubt it.
Up to this point, I have enjoyed using the Roku Stick (and my four other Roku devices); however, it may just get to a point I convert over to Apple TV.