I've got Cynosure twice, one T4 and other T5, both had only one row perks. I guess this is because of the old reset system of vendors. Previously, multiple resets of gambit/vanguard/crucible vendors gave a chance for additional perks for weapons that drop from respective tiers. I think the system got reenabled with reintroduction of vendor reputation and now conflicts with weapons tiers; the system should be disabled to fix the behaviour.