The bug
Horses, donkeys and mules can enter boats when they're babies. While inside of the boat, they can grow up, leading to adult horses in boats. As the hitbox of an adult horse is larger than that of a boat (see MC-89963), the boat can then only be entered from below.
To reproduce
Place a boat
Spawn a baby horse on top or next of it, so that it enters the boat
Feed the horse with haybales
→ ❌ An adult horse sits in the boat
Original description
I played with friends and found a mistake. If you put a little donkey in the boat, and he grows up, then you will not be able to break down or get into the boat without killing the donkey or flooding the boat.
Linked issues
relates to
Attachments
Comments

Can confirm in 20w49a. I simply placed a boat next to a baby horse and reproduced this issue using the following command:
/summon horse ~ ~ ~ {Age:-100}
Can confirm in 21w06a.
Can confirm in 1.16.5 and 21w08b.
Can confirm in 1.17.1.
Can confirm in 1.19.
Fixed. most likely in 23w07a along with MC-256465
Confirmed for 20w06a.