There's no way it's not a bug. The catalyst itself says to get kills with the "Perfect Fifth explosion", but that doesn't count towards its progress. If it's been like that since Warmind launched them it's simply a bug they've never been notified of, which is understandable: catalysts are very rare, as are the number of people who actually use Polaris Lance. And I have tried grinding it out in the Castellum as well as EP, The Ace in the Hole mission, as well as other areas where lots of ads spawn and clump together. If someone official from Bungie replies that it's not a bug then I'll be even more annoyed than I was before.