The bug happens when you have a lot of cows in a fenced area and you breed them. When the babies grow up they glitch into the fencing and slowly get pushed out.
I've tested this out a few deferent times and it happens pretty frequently. As far as I know, it's only with cows and in 1.17.1.
Related issues
Comments


Thank you for your report!
We're tracking this issue in MC-9568, 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
-- I am a bot. This action was performed automatically! The ticket was resolved by one of our moderators, and I left this message to give more information to you.
Duplicates MC-9568 (see MC-2025 too)