While I was having fun with the new "Deferred Technical Preview" stuff, I tried to place an oak boat and it randomly disappeared. I don't know what caused the issue, I placed the boat multiple times in the same world with no issue.
Update: I have done some research and found that the boat only disappears when you try to spawn it too close to a solid block, anywhere 2 or more blocks away from a solid is fine.
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We're tracking this issue in MCPE-177283, so this ticket is being resolved and linked as a duplicate.
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