Mob outside the World border are sometimes displayed as being at the World border. Use /tick freeze and then move on the border to reproduce stably.
Here is a simple reproducible example:
1. Create a world with seed -8914574169295752653
2. run /tick freeze
3. run /tp 29999998 67 29998541
Following the description, I could not reproduce any unusual behavior. Please add steps to reproduce the issue, and a video showcasing it. 🙂