I hope this helps everybody. I was having the same issue, so I decided to do all Naval Contracts that I had left. I did that, and a notification popped up on my screen informing me that the last contract at Serranilla had been unlocked. You have to complete all of the other contracts to unlock the contract at Serranilla. Hope this helps.
there is no glitch...as has been said time and time again if you care to read, you do all other navel contracts and you get that one last its called the final contact or something, as for the assassin contracts, the only one ive had an issue with is that one from salt key or whatever its called, it failed and the ship appeared then it vanished and never appeared again, altho i had one where i had to kill brothers, one ran away and i got stuck fighting so i failed it but i fast travelled to a nearby place and it re appeared.....had no other issues though
Having the same problem with this bug as well, thought doing all the other naval contracts would have the last one appear but it's not still and I gotta say it's a let down....... so please fix it!!!!!!!