I could be the settings you put when you render the file. I had the same problem, but on Premiere Pro. It was "fixed" when I used different settings. Try looking up what the "optimum" settings are for saving for YouTube (if you haven't done so already) and try that. But if you are already using what people recommend for YouTube, then I am not sure.
I suspect it would be some codec-related problem, but I'm no expert at that.
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