It happened twice, so I'm sure it bugged. The boss of the Shipwreck Strand suddenly disappear after took 2-3 hits from player. When it disappeared I got my achievement but I got no loot from it because it doesn't die yet. Just disappeared.
This also happened to me earlier today. For me, it was almost dead, but it disappeared before I could kill it, and I received the achievement but there was (of course) no body to loot.
Please can we get some sort of comment from support about this issue.
It burrows and moves to different locations. It might be bugged but not sure if you are aware of this. Fought it a few minutes ago and died. Respawned and waited while some higher level guy came by and killed him. Seemed fine to me.