The mobs in the nether are able to spawn outside of the world border. This includes not only the new Piglins and Hoglins, but also old ones such as Ghasts and Blaze.
Also, the nether mobs are able to pass freely through the world border on both sides. I would assume overworld mobs work for this as well, but there is no issue in the overworld, and zombie piglins, etc. do not have this issue while in the overworld.
Thank you for your report!
We're actually already tracking this issue in MC-121997, so I resolved and linked this ticket 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 in the future to see if the issue has already been reported.
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