Why The Boat Could Be Disappeared For Me In The Level 61 Quest?

when I complete the stage 4,it teleport me to the island that was the last,I had collected the strings and planks,when I get on the boat,the boat did not work and it was disappeared,the worst point is that you can not leave the island unless you type the command /kill

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Stage 5
» Leave the hidden island and stop the pirates from stealing Redbeard's Booty.
Dialogue:

• A key is laying on the ground, it seems there were more than one. Better take it and figure out a way off the island.

Pick up the Ancient Key near the middle of the red "X", then head over to the boat at [25, 38, -15260].

• Maybe you can fix this boat and get off the island?

Spoiler alert: The hidden text contains spoilers relating to leaving the island .
You have to fix the boat with 3 Strings and 4 Wood Planks.
The strings are located in a cave.

Location Cave
90
45
-15210
The planks can be found underneath a tree on the island.

Location Tree
80
39
-15255