Is this similar to DNR schemes? Because that's a lossy process that reduces find detail, and is notorious for having been misused in many bluray releases of analog film remasters.
And adding digital grain "back in" doesn't sound very authentic to the original analog source.
Film grain is so expensive to encode that in bit-rate-constrained situations you would be forced to choose between obvious ugly compression artefacts, a reasonably good image without grain, or a reasonably good image overlaid with inauthentic grain.
Besides, "authentic" is such an overhyped pile of poo anyway. Everything is a reproduction; what matters should be that the artistic intent remains unaltered.
And adding digital grain "back in" doesn't sound very authentic to the original analog source.