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...
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