Sorry, my bad...this issue is probably already resolved and I have got the wrong version. Everything is working as intended now, I believe...
It seems that this wasn't a bug, after all, it just seemed the mobs that spawned below the nether bedrock floor were taking up the mob cap. I just went to almost build height limit and came back down and they were everywhere. Thanks!
Sorry, my bad...this issue is probably already resolved and I have got the wrong version. Everything is working as intended now, I believe...