Originally posted by Veseljko:

Air pens dont work anymore. One with elevated ground should work if you made it properly. I use it for both, boars and wolves and it works fine.

I think I might have found the issue -- in a thread it mentions making sure the pigs face north as that is the orientation they spawn at on load. Just curious as to why they would glitch out to the fields -- shouldn't they have spawned in but been trapped in the room surrounding the breeding pen?