Boar Breeding

So, I get a few boar together in a pen, feed them,. they become tame and I have piglets. However at somepoint they will glitch out of their pen and I'll find them roaming around near the base. I know they are mine as they are still "tame". The pen is solid wood with no gaps so I've no idea how they get out. I did witness them glitch-flying once. Also, could there be a way of "leading" the tame ones? Getting them back is taking me forever...