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.
I'm unable to reproduce this, did you have the gamerule doMobSpawning set to false by any chance?