I cannot find anymore monster mobs after I created the Nether Reactor

What I expected to happen was...:
After I built the Nether Reactor, it turned to night. I expected to find monster mobs (e.g. Skeletons, Creepers, Spiders, Zombies) across the rest of the map.

What actually happened was...:
After I created the Nether Reactor, it turned night and no more monster mobs were found on the map (e.g. Skeletons, Creepers, Spiders, Zombies).

Steps to Reproduce:
1.Create new survival map
2. Play and find resources to build Nether Reactor (diamonds, iron ingots, gold ingots, cobblestone)
3. Build the Nether Reactor
4. Activate Nether Reactor
5. Collect items and kill Zombie Pigmen
6. Knock a hole in the Netherrack wall.

It will be night time and no monser mobs (e.g. Skeletons, Creepers, Spiders, Zombies) will be present