Grain manipulation at proxy resolution should be avoided as the results are unreliable. The grain selection area may be too small at proxy resolution to give a good result, and making this area larger may drag in unwanted detail from the image. If you try to use F_ReGrain at proxy resolution, we simply pass the image through untouched and issue the following warning:
Cannot work at proxy scale.
We decided that this was preferable behaviour to doing poor grain replication at proxy resolution. You can, of course, crop the input clip and work with that rather than the proxy. There is a minimum size for the selection box, which is about 37x37 at base resolution. If the box you select is smaller, you will get this warning along the top of the viewer:
Sample box is too small - please select a larger sample of the grain.