We just killed a Korteth and 3 people had already locked in damage wise so i just killed it to get it off the tab and it gave all four of us an xp and artifact bonuses like we were all locked in.

It is a 4 person NPC. There are a couple of them.

The lock symbol merely guarantees you a reward if more players have hit the alerted NPC than there are available rewards.

Since the Korteth is a 4-person NPC, it can provide 4 rewards. So if five or more people hit it, only the top four damagers will receive a reward; the person in position 5 will not receive anything. The game will also automatically give you a reward if you're in a reward position regardless of whether you have locked or not. However, the game may not always give a reward if the damage caused by a "top damager" is equal to less than 1% of the NPC's strength, so make sure you always cause at least 1% damage and make your way into the top damagers to guarantee yourself a reward.

I hope this explains the way it works. What you've described is not a bug, it's the way the game works. Oh, and have fun with the Korteths; they're easy kills with good rewards. Later on, you'll encounter 12-person NPC's that are a #$&*@ to kill.