When walking away or towards any passive mob the mob will disappear from a certain distance away. Walking closer or further will make it reappear. It very rarely happens.
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migrated
Auldrick
It has been true for a long time (since before 1.0) that entities—including not only mobs, but minecarts, boats, beacon beams, etc.—are not rendered beyond a line of sight distance of around 70 blocks. As you (or the mob) approaches or retreats, it appears and disappears suddenly. Is this what you're describing, or is it happening much closer than 70 blocks distance for you? Also, what is your render distance set to?
migrated
This only started happening for me since the 1.2.13 update. It is happening much closer than 70 blocks. The mobs don't appear for the first time until I'm within a couple blocks. I'll go to a place I know I have mobs in a pen and until I'm almost on top of them, it will look as though they've all despawned. I'm also experiencing this with hostile mobs (see MCPE-32231), again only since the 1.2.13 update. My render distance is set to 12 chunks.
CrampedSpider72
This is intended since loading up too many entities can crash your world. Imagine your render distance is set to 64 and all those entities are loaded(can walk, interact with stuff), this would create a huge lag and potentially crash your world. Also what is your ticking range set to? The longer it is the better you'll see
[Mojang] Mega_Spud (Jay)
Thank you for your report!
However, this issue has been closed as a Duplicate of MCPE-32038.
Your additional information and observations are very useful, so if you can add them to the comments section of the parent issue it would be appreciated.
Please add a vote to MCPE-32038, and if you know of any steps to reproduce the issue reliably please add them there too.
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I have noticed this as well since the 1.2.13 update playing on Windows 10. It is happening for passive mobs including villagers, horses and cows. I am seeing this almost every time I approach passive mobs. I don't know if this is related to the same bug, but I am also getting dismounted from horses very frequently for no apparent reason since the 1.2.13 update.