You need to "fence them in" once you caught them. Sometimes - most often when there are too many of them in a certain space - they will slip through whatever (normal) barrier you have build around their "spot". Yesterday we found a tamed 2-star boar outside of our Farm watching us trying to get the fully loaded cart over a small bridge we build - after a few mins i went and had a look why it did not attack us - it was one of ours :)