I have finally had a chance to fix this bug (let me know if you find otherwise.)
Yeah, all the rocks use the same scoring mechanism, and initially I was a bit confused, because I definitely had logic at the top of the procedure to set the rock's score to zero if it wasn't on the board.
I spent a while thinking that somehow how it detected 'on the board' wasn't working. Finally, I realised that while I was setting the score to zero for these rocks, I wasn't actually stopping it from carrying on through the rest of the scoring routine anyway!