Thanks for the post.
This may be related to a hardware issue. Have you contacted your TV's manufacturer directly to report this issue and request additional support?
You can reach them here: https://support.tclusa.com/televisions or at 1-877-300-8837.
Actually, this looks more like an issue with Dynamic Contrast. I know that on my Sharp TV, I would occasionally see odd abrupt brightness changes (full screen) until I turned Dynamic contrast off. And so, I would suggest to those seeing this particular issue, turn off Dynamic Contrast and see if that fixes the issue.
Now, since Dynamic Contrast would appear to be a function under control of the Roku software, another problem that seems to need solving is the habit of Roku technical posters to just point people with problems to the hardware manufacturer, instead of attempting to truly isolate, understand and then correct the problem.
Perhaps you could point to my previous paragraph as a "Solution" and Check mark it accordingly.