I tamed a parrot, and caught it in a boat to take home. I noticed that leaving the boat (near land) would cause the parrot to also exit it, so I tried to make the whole trip without exiting.
Unfortunately I had to exit the boat (far from land) anyway, but instead of simply exiting the boat, the parrot was gone.
At the same moment, the server console said something like: "[Server thread/WARN]: Keeping entity Parrot that already exists with UUID"…
MC-154068 parrots occasionally disappearing when you take them from a boat