Happened to me as well with my Pirate/Double Agent combo moving away from the Navigator/Helmsman it was targeting after the latter changed its course a few times - reported it as a bug.
Edit: It is definitely a bug as the map is showing the outpost where my Pirate is headed to (Champino) as belonging to my opponent.
Champino is mineChampino is Squiid'sEdit2: I think I found the bug. At one point, my opponent should have targeted Champino - the time wheel would show that he is conquering it. After changing course again, that parameter is not reset and the software still continues to believe that my enemy's sub is headed to Champino.
Thus my Pirate sub continues to follow the ghost enemy sub to Champino, instead of rerouting. After my opponent's sub finally reaches its destination, the parameter is reset and my sub heads to my nearest outpost.
Nothing to do with speed or the map being round.