Both Mobs and Entities despawn when world is loaded, even though the mobs/entities have been tagged.
I encountered this bug on Dec 23, 2021. When I loaded up my survival world, These mobs/entities despawned:
*Hopper Minecart
*Cows(w/ Name Tags)
*2-3 Oak Boats
*An Enderman
*A Drowned
This bug makes survival worlds unbearable and annoying to play, having to gather all the mobs and materials to fix the losses of these mobs/entities every time I load up my world
Related issues
Comments

Thank you for your report!
We're tracking this issue in MCPE-144208, so this ticket is being resolved and linked as a duplicate.
If you would like to add a vote and any extra information to the main ticket it would be appreciated.
If you haven't already, you might like to make use of the search feature to see if the issue has already been mentioned.
Quick Links:
📓 Bug Tracker Guidelines – 💬 Community Support – 📧 Mojang Support
📓 Project Summary – ✍️ Feedback and Suggestions – 📖 Game Wiki