I cannot confirm nor recreate this issue. But I've been playing in a hardcore world on the newest snapshots (with and without LAN). And constently there are no mobs to be seen, plenty of friendly mobs, like, A LOT. But very low amount of hostiles. 

While typing this I don't think this has anything to do with hostile or friendly mobs, because there are no bats spawning either. 

• Render distance doesn't change anything.
• Killing all mobs using "kill @e[type=creeper]" for creeper, zombie, skeleton, spider etc. doesn't help, some respawn ofcourse, but mostly nothing. Mostly ~30 per mobtype killed.
• I went in spectator mode, to a place that didn't have torched caves. Executed the command: "/effect give @e minecraft:glowing 30 1 true" and the result is in the screenshots.

I am unsure if this is an actual hardcore bug or not. I am playing in hardcore. I started a new world (non hardcore, set it to night using "time set 18000" and ran the same effect command. The result is in the screenshots