Mae

This is happening for my realm in 1.17. Backing up the world to the latest option in World Backups doesn't prevent it from losing 30 minutes of progress. 

Encountered this issue with 2 named dolphins. They are in a fully enclosed tank with 2 air blocks. The tank is located near the ocean where multiple dolphins spawn regularly.

Version 1.14.4, multiplayer server. Left them unattended for a few in game days and they disappeared.