Boat disappears when on water above soul sand

Hello,

I put three blocks of soul sand under the water to make a water canal boat elevator. When I rode my boat from east to the west, and back down, several attempts were successful. On another attempt, I probably stayed longer on the soul sand. Then I moved on. My boat got stuck when I wanted to leave it. I pressed Shift once more and left. Then I tried again, and lost my boat above soul sand. I went sleep, and didn't see it in the morning. I restarted game and it was still lost.

Pavel