I've been playing the same world since the beta and I've noticed some odd friendly mob behavior. At first, while I had barely explored my world, mobs were in abundance everywhere I looked. (Exception wolves which are not showing anywhere and parrots and ocelots which I don't see because I have not found a Jungle biome yet). I was surprised at how many there were of each type in such a small area (10+ of each in about a 100x100 area).

As I killed them for food, resources, etc. I did not see their numbers diminish.

However, when I began seriously exploring the area (after completing a 2048x2048 map) I realized their numbers had been dropping off the more I explored and returned to base.

In the new areas I explored friendly mobs were everywhere there too. Not quite as high in numbers, but that could be because I was mapping and therefore moving quickly through sections as opposed to staying and exploring. The point is, new mobs were spawning regularly and in good numbers.

Back at my base now, the friendly mobs are trickling in now. Where I had large numbers, I look around and see under 10 in total. The number fluctuates a little but the numbers are way way down.

I'm not sure why I'm not having such high mob rates anymore. I would not expect exploring to diminish that number. When I am no longer in an area I would expect everything to despawn and not count towards any world caps that might be set.

So it definitely is an issue. But not sure why it is happening. There's nothing else I am doing that could explain the issue.