Interesting, makes sense. Now that I think of it, the bridge scenes in both episodes didn't have nearly as bad grain. So we can look forward to a variety of film grains throughout the series, reflecting how each scene was shot. I.e. it's not a bug, it's a feature.
Well with Inner Light they were obviously trying to create a soft and dreamlike mood in the planet scenes, to reflect the fact this was a thousand-year old memory we were watching.
Making the picture a bit grainy
was probably the quickest and easiest way to do that, and never would have been detected in the original SD broadcast.
Anytime they go to a planet with the same soft kind of lighting, we're probably going to see the same grain show up.