Any Grain Intensity > 0.0 leads to jitter. The Grain Jitter amount or unchecking on/off does nothing?
You might be seeing temporal AA dithering jitter. In the project settings, switch the AA method to something other than TAA and see if it still does it.
Not to bring up an old thread but I have the same issue. Jitter has no effect at all. Currently combing through the engine source to see if it’s a bug.