TU47/CU37/1.40/Patch 16, TU48/CU38/1.41/Patch 17, TU50/CU40/1.43/Patch 19, Past Release - (TU55/CU46/Vita 1.53/PS3+PS4 1.54/WiiU Patch 25/Switch Patch 5, Past Release - (TU56/CU48/PS4&PSVita 1.55/PS3 1.56/WiiU Patch 26/Switch Patch 6
Xbox One, PlayStation 4
Update: Although the behaviour seems to be happening more on chunk edges a similar mob pushing behaviour seems to apply upon loading as well causing suffocation or escaping the enclosure.
Update: Added video of a larger enclosure and image: Spider killing a villager by suffocation. Seems that mob pushing does increase the chance that an entity ends up in the wall or escapes it. This video took 3 reloads for this to happen it's not something very rare.
Update: This behaviour seems to apply to every entity incl fenced in animal. I was able to replicate it with cows etc this way.
Mobs / villagers glitching through blocks upon load. (chunk edges)
When villagers / mobs are kept in a space across two chunks they glitch into the adjacent block when loading.
To recreate make a enclosure as can be seen in the video one half on one chunk the other half on another chunk.
Reload the game while standing in multiple chunks near the enclosure.
They glitch through the corners. (depending on the loading algorithm and the chunk you are in)
On crossings/intersections of chunks they glitch out in all 4 directions upon loading not at once (again depending on the chunk you are in).
This probably explains why people have villagers in glass or mobs escaping from mob farms. (I would not like this happen to my creeper farm)
I had all my villagers escape my iron farm every time I load some glitch through the blocks (eventually ending up with a empty enclosure). I build this is a plot where chunks are laid out.
Only on the chunk edges this seem to occur. I build the exact same farm in one chunk (as it is designed to be that size) no villagers escaped.
I guess this is related to mobs being loaded before the chunk as I observed when loading a game multiple times. You sometimes are able to see the mobs in caves before the chunk loads/renders. It's not only related to multiple mobs pushing each other, as they will eventually glitch out till there is no mob or villager left. A 4x4 cell seems to have the same behaviour.
Screenshot is my second time escaped villager first missed the lava blade now goes through glass. Placed on the intersection of 4 chunks.