This is an old "bug". Back in the day as long as someone was on The Splintered Throne - Quest - World of Warcraft, whether or not they are done with the quest, you could master loot or need on the shards to someone else who isn't on the quest.
In other words, as long as someone is on The Splintered Throne - Quest - World of Warcraft, any plate class can loot Shadowfrost Shard - Item - World of Warcraft via need/greed or master loot. I tried a quick search for it on here, but nothing came up. But it has been mentioned on wowhead ( Shadowfrost Shard - Item - World of Warcraft ) before. So defo not new.