I'm assuming it's a bug since every season has the seasonal exotic in the guardian ranks, but if not that's really classic Bungie tying our guardian rank progress to dungeon exotic rng! Some people have gone 100+ clears without getting certain dungeon exotics and lots of players out there don't even play dungeons/raids. Every design choice this season has been about slowing our progress and adding more grind/busy work (weapon crafting, potions, pinnacle cap, etc) so we keep playing more. I wouldn't be surprised at all if this is intentional.