I would concur with that, if it is not affecting ALL sources, and especially not affecting the vizio setup menu, it is content related and really nothing to worry about.
Sometimes you'll see odd "artifacts" like that as the result of compression on digital movies and it can even be occasional from the adaptive backlighting and the ambient room light sensor being a bit off. You could very well watch the same movie again in the day time and have no issues.
Unless you see it happening more frequently or getting worse, I wouldn't worry about it.
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