This issue may occur because the sharpness in Vivid mode is higher than that in other modes. Please use Cinema mode instead of Vivid mode when the content shows grainy picture. If you still prefer to use Vivid mode, please following one of these ways to solve the issue:
(1) Reduce "Pixel Enhancer 4K" in OSD menu (OSD >> Advanced >> CinemaMaster >> Pixel Enhancer 4K).
(2) Adjust "Sharpness" in OSD menu to "8" (OSD >> Picture >> Sharpness).
Parameters for each picture mode are independent. The default value of sharpness in Vivid mode is higher than that in other modes, so the content tends to appear sharper in Vivid mode. When sharpness is too high for some content, the grainy picture noise may occur.