Can confirm on the Better together update hostile mobs are not despawning. I have had the same skeleton outside my wall (and spider outside another) for the past week. Even when traveling many chunks away and after leaving the world. Setting the game to passive is the only way to despawn all the stuck mobs. Slime farms then work on the first visit but subsequently stop working after first harvest as the mob cap is quickly reached again in the surrounding area. On the flipside, passive mobs (even ones bred and kept in pens) seem to randomly despawn. Seems backwards.
This has been occurring on my Realm with my buddy. I play on Xbox One and he keeps getting intermittent disconnects form my Realm. This occurs on both his PC and his Xbox One. This also occurs on his cousin's Xbox One, who lives in another town.