Update: I tried to reproduce this bug and I wasn’t able to get it to happen, but then with a different angle it did happen. It appears that it only sometimes happens, and quitting and reloading the world fixes it.
The mannequin’s head is locked in a direction when out of a boat. While inside a boat, the head position is locked in relation to the body, but upon leaving the boat, the body will still remember the original direction and act as if it is still facing that direction while the head will become locked to a new direction. So, the mannequin’s head is locked in a direction at all times, but can be put in unnatural poses by being pushed once its direction is changed using a boat.
This is still an issue still in 19w04b. If you are in Survival, you can still get to the islands without cheats by using a slime block flying machine but there will still not be much of a reason since the end cities are not there.
This is very similar to MC-301810. Seemingly the body rotation is desynchronized from the head rotation on the client side, and doesn’t register the change of the head rotation for some reason.