Steps to reproduce:
1.Create a nether portal or end portal.
2.Do some inventory changes.
3.Lead a boat/boat with chest to the portal, until it disappears.
4.Do some inventory changes different from Step #2.
5.Wait for the game to crash.
6.Reopen the world and Step #4 will not be saved.
Can confirm; the issue isn't specific to boats but to any leashable entity.
The following command reproduces the issue: