Noticed this mostly on realms. I am owner of the realm. I play daily. I had a secured village, rather tall wall, no openings, and approximately 15 villagers and an iron Gollum. I log off properly. I log on next day and the village is down to 3 villagers and no Gollum. I repopulate the village to over 15 villagers ( I have beds for 20+) and make 2 Gollums. I log off properly. I come back the next day to 3 villagers and no gollums. No signs of attack. No villager zombies and wall is intact. I also had about 30 adult chickens in a fenced area. They are down to 2 adults and 5 chicks. Others have reported same thing on minecraft forum.
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:
📓 Issue Guidelines – 💬 Community Support – 📧 Mojang Support – ✍️ Feedback and Suggestions – 📖 Game Wiki