Bug/glitch?

Has anyone experienced a bug where another pirate has the death sound but no animation and just disappears. Then respawns on your ship? Yesterday while sailing on a gallon with a full crew there were two pirates that kept reappearing on the ship from thin air. At one point i watched the one appear. Has anyone else experienced this? at one point after defeating them we sat there for 10-20 minutes watching the seas and ladders for them to come back on their sloop but then was caught by surprised when myself and another member were struck down in the back.