The Murcko Scaffolds node produces skeletons with all the carbon atoms numbered with 12, I can't recall I have seen this before. Changing the column renderer does not make a difference. This is in Knime 2.12.0 with the latest RDKit nodes (2.5.0.201505221301).
The "12s" that were showing up were due to a bug in the code: it was assigning an explicit isotope to the carbon atoms in the skeleton. We've fixed this and the updated code is available in the RDKit nightly build.