Looks like a couple of people have noted this in other forums, but I thought I'd add it here with the other bug-reports.
Basically just what the title says. Fletcher Corbin Day, at the livewood sawmill that Roost and the Lone Wolves have occupied north of Driftwood. After clearing the sawmill you can invite the Fletcher to come to the Lady Vengeance. He agrees to join you, but then he just stays in the same place at the sawmill and never shows up on the ship.
If you talk with him again at the sawmill the dialogue is like he is already on the ship; he talks about how it's nice to join you here on the seas, etc. But he's still at the sawmill.
I tried nearly everything to get this to trigger correctly – tried to control his position during/after the fight, tried to clear out surfaces between him and the exit, even went back and entered the sawmill a different way and did the fight before confronting Roost. Nothing seems to work.
Hopefully it is something that can be easily fixed, because I'd love to have him with me on the ship!