I recently started using a Fuji GW490 with Portra 400 film. Having got the film developed, some of the areas out of focus (or smooth gradient areas) appear to have a 'splotchy' pattern of lighter dots that doesn't necessarily look like normal film grain. Perhaps the film is underexposed/expired? Possibly it could be how the lab handled it as I had a few other rolls come out fine.
The diffuse dot patterns look like emulsion reticulation. It is damage to the emulsion usually caused by heat during development, but can also be an artifact of old film.
They don't look like scanner artifacts because the dust and hairs on the scan are perfectly sharp.
Have you looked at the original with at least a 20x loupe? Reticulation would be evident at that magnification.
If you can see something through the loupe, you can try cleaning the emulsion with PEC-12 and re-scanning. It's unlikely to be dirt from the looks of it, but you never know.