Lately, I have had horses, villagers, polar bears and mooshrooms that were enclosed on chunk borders disappear and we have not been able to find them. They were enclosed in pens where they were not able to get out and that were smaller than 20 x 20.
Related issues
Comments

Thank you for your report!
We're tracking this issue in MCPE-21416, 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 – 📖 FAQs
-- I am a bot. This action was performed automatically! Please report any issues on Discord or Reddit