I play on PS4. I just got the 2.18 update a couple of days ago but this was going on before that and is still occurring after. I keep several animals on a farm. All are passive or neutral. I have one of each color of sheep in a pen that is less than 20 blocks in any direction. I noticed recently that several of them had disappeared so I replaced them. Then several had disappeared again the next time I played. So replaced them again. When it happened a third time I named all of them with name tags. It has now happened yet another time even though by several rules they should not have disappeared. This has also happened with ocelots and polar bears. Several of these were close to each other and there had been a thunderstorm so I thought that was possibly the culprit so I put a ceiling over them. Then more named animals in pens disappeared from under the ceiling. A lot of work went into bringing these animals to my house and it is extremely frustrating for all that time and energy to have been for nothing.