Endermen can teleport off of soul sand, even if not provoked

I want to make a mob zoo, but since Endermen are now allowed to teleport out of boats and minecarts, I don't have an easy way of containing them. But then I found out that they shouldn't be able to teleport of of soul sand, unless they are provoked. (According to https://minecraft.fandom.com/wiki/Enderman#:~:text=44-,Teleportation,exclusively%20at%20the%20teleportation%20destination. ) However, when I tested this, they were able to teleport away. If this is intended, how else can I contain an Enderman? (Other than making a large area around it covered with water).