Repro steps:

1. Place 3 hoppers facing north
2. Place a hopper one block higher, facing north, but one block south of the other three
3. Place a chest boat on one of the lower hoppers
4. Hop in the boat and ride it so it is on top of all three hoppers
5. Sign out/In
6. Boat moves a little backwards

After reload, you can ride the boat forward again and if you log out again and log back in, it goes a little backwards again.

Once the boat has gone backwards enough that can't unload items into the first hopper, it stops moving:

Sample world file attached, you can ride the boat forward, log out/in and see the boat going a bit backwards from where it was left. It seems like a floating point or rounding issue.