What I expected to happen was....

After killing all the passive mobs in an area, they would respawn over time and/or when I left and reloaded the area

What actually happened was...

After killing all the passive mobs in an area, they failed to respawn under any circumstance. I was in the area over several in-game days, and when that failed I left the area for a few minutes to reload the chunks. I closed and restarted the game several times, and have toggled between difficulty levels and even creative mode. Nothing has changed.