Thank you for solving the issue! YES there is indeed a chunk border line between the boats. The issue is solved after placing both boats in the same chunk! This is why the boat was sliding off...
[media]updated to attach steps to reproduce and expected vs observed
EDIT: The video shown is only an example of how the boats slide off. I have tested this issue in both fabric and vanilla servers. BOTH SHOWS THE SAME RESULT!
Can be reproduced in 1.20.6 vanilla server but cannot in 1.20