Spectator Mode Summoning

When I start a command block summoning zombies on a continuous loop in spectator made them brieflyappear and the disappear.

What I expected to happen was...:
The Command block would comtinue spawning mobs when I changed gamemode

What actually happened was...:
The Zombies briefly appeared and the disappeared. Nearby Villagers continued Without getting hurt. When I change back to survival they started spawning again.

Steps to Reproduce:
1. Open a super flat world.
2. Find a village
3. Just outside it put a command block with the command:
/summon Zombie
4.Attach it to a loop
5. They start coming but then change gamemode:
/gamemode 3
6.They stop spawning
7.Change gamemode back
8. They start spawning again.