Ah - so after 2 days of trying to figure this out, I finally ask SO, and what happens? I figured it out ten minutes later...
The issue was in my drawable.
What I forgot to mention, is that I'm using a second RatingBar, overlayed on top of the first one (with just empty transparent stars), as I'm using a colour filter on the primary one in order to change the colour of the stars based on the rating (red, yellow, green).
What was happening is that the drawables I was using for the primary RatingBar needed to use the exact same ones as the overlay - and not the ones I was using that were left over from my previous theme...
So, it all comes down to the images used, not the code. I'll accept this as the right answer once I'm allowed to.