Mac OS 10.13.6
This bug only came about in 19w13a, mob spawners worked properly in 19w12b.
In order to reproduce,
- Find a spawner (blaze, zombie etc.)
- Clear around the spawner so there are no blocks, at least 9x9 area.
→ You will see spawns only happen on the spawner, if there is no block blocking a spawn from occurring.