I've been transporting a TON of villagers on boats + leads (each boat has two villagers). I have a ton of villagers waiting in a field, they're all in boats, and every boat is full and has two villagers. I went away from that field, and when I go back, some villagers are able to escape from their boats, wandering around. This has happened multiple times on 1.17.30, but I've never encountered this bug on previous versions.

Within that field, there are no job sites or beds nearby, although sometimes I'll put my bed and sleep nearby. I always sleep during night/thunderstorm, so they should be safe. The boat itself is also intact and not destroyed, so I usually push them back into the boat if they start wandering around. This might be unrelated, but their trades and jobs have been locked, and they've been zombie cured 5x.

This glitch seems to happen when I slowly approach that field, with about 10% chance. As that area is being loaded, the villagers can somehow get out of their boats.

Most of the time I put villagers on boats, but this glitch might also occur on other mobs as well.

Update: Still a big issue in the latest version (1.18.2). I was out exploring in the overworld, and when I came back, my villagers are somehow getting out of their boats!

Steps to reproduce [by EVGENSYPERPRO]:

1. Load the world MCPE-142983.mcworld.
2. Enter the Nether portal (it is built nearby).
3. Exit the nether.
4. See how some of the horses got out of the boats.

1. Load the world MCPE-142983.mcworld.
2. Teleport to coordinates 0 100 0.
3. Teleport to coordinates 218.04 78.12 10001.09.
4. See how some of the horses got out of the boats.

Observed results:
Mobs can get out of boats under some circumstances.

Expected results:
Mobs without the player's help should not leave the boats.

Video:
Minecraft 2021-11-22 15-54-04.mp4