bug mobs when they are in enclosure

Hello, In one part survival, when I create an enclosure has mobs, everything passed well after 5 minutes and expected to be part of various other activities, I noticed that the mobs smells coming out of the pen vallable no reason, I'm sure I put barrier and they do not have any opening anywhere, I want to clarify that I have seen in several forums and the concern was not me, but several individual when you move away for some time, eventually leaving the mobs came out of the enclosure, that is cows, pigs, chickens or sheep, the problem remains the same and persist.
Sincerely.