I found a spawner that was "open" on one side of the mountain slope. The problem is that after walling, the light is still inside. Certainly there is no hole through which the light enters, and what's more the zombies set themselves on fire. Lighting changes with the time of day.

Third screen is another world with same seed, same spawner. Everything is working fine there.

Seed:  207312516180738410

Coords:  -8921, 688