This is for the most recent version of Better Together.

I noticed mobs don't de-spawn, mostly with hostile mobs, but also passive mobs.

I walked through a jungle and there was a blue parrot on my pathway. Then, after going a full map distance away and returning, the parrot was still there. Even the day after I reloaded my world, the parrot was still there (untamed).

Hostile mobs seem they don't de-spawn either. This problem was most prevalent on a Realm world, but also seems to exist on local worlds. No matter how far you travel, and how much time passes, when you return to the location, the mobs are still present.