I haven't tried an encode with your settings, but checked the effects of the added grain.
I think there's too much grain compared to the original, tbh. It may probably work but mainly because if the grain is always present throughout the stream, the problem is not "triggered" at all (i.e. x264 always keep the quantizers at a certain level).
I see this more like a way to "work around" the problem rather than to fix it.
It may work now, but I can't add grain to every stream...
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