Put the summary of the bug you're having here

What I expected was... going near a mob spawner will have mobs spawn around the player in empty blocks rather than in solid opaque blocks

What actually happened was... mobs will only spawn inside opaque blocks and even after some testing the game prefers to spawn them inside opaque blocks rather than glass, slab, or stair blocks.

Steps to reproduce:
1. Find a mob spawner
2. Place opaque blocks around spawner
3. Wait for mobs to spawn inside only the opaque blocks
4. Try same experiment except with glass blocks