I have a similar issue with a 2 star boar I harpooned home.
My best guess is that his "frightened run back to base, player too far" code is active at the same time as the "tamed" code.
Not entirely sure why. Personally I dragged the wild boar home before taming it. Not sure if that's a factor or not. But it's the only boar in the pen that does this.
(And I refuse to kill him. he has personality!)