what are your render settings? show us in detail.
It seems like resizing when exporting is solving the problem. Here's a screenshot of my UI and Export settings that yield grain in the export.
The viewer's at full resolution, which doesn't seem to affect it. The main thing seems to be that when I export to Prores at original size or at 4K the grain is there, but when I resize it to a width of 1080, and export to Prores or even H.264 it looks way better.
I'm not using a high res monitor so maybe that's part of it. Interesting.
when you export, do you use Animation or Prores?
also, how are you viewing the result? import the footage back to Ae - this should be the visual test if there is any difference.
one case worth mentioning: I once had a bug in my custom render settings that made the video softer looking (also CC2017) the fix was to trash my custom render settings and make a new template
I've been using Prores to export. I've been looking at the video results in quicktime, but I did import the video file back into AE and the grain is still there. It seems like resizing is fixing it. Still seems like some sort of bug though like you're saying. I will definitely try deleting the custom render settings and starting fresh. Thanks a bunch.
Please return and let us know if that worked for you.