Animals glitch through blocks

All mobs keep glitching through blocks, this makes it extremely hard to get food. For example: when a horse is in a pen it just walks out through blocks. I have tried numerous experiments and they have all had the same effect no matter what mob or what block. I first discovered this a few hours after I got the new update, I thought that the mob would stay in the pen but a few seconds after I took it off the lead it just walked through some fences and continued walking.