Covering roof of spawn chamber with bottom-half slabs severely reduces the spawn rate

Everyone knows that mobs cannot spawn on top of bottom-half slabs. But in this specific case mobs hardly spawn at all below solid opaque blocks with a bottom-half slab on top of it. This somewhat relates to MCPE-12422 and now-fixed MCPE-11112 but is clearly not the same.

Steps to reproduce:

1. Build a 12x12x3 spawn chamber out of any kind of solid opaque blocks.
2. Optionally place tripwire and redstone lamps to ease observing mobs inside it.
3. Take 24 blocks away from it and wait 10 seconds or so.
4. Observe that mobs start to spawn in the chamber.
   
5. Now cover the roof of chamber with any kind of bottom-half slabs.
6. Take 24 blocks away from it and wait for mobs again.
7. Observe that the spawn rate has severely reduced. Now it takes several tens of minutes to spawn even a single mob.
   

What I expected:
Slabs on the roof do not affect the spawn rate at all.

Note that the inner height of spawn chamber needs to be 3 blocks or higher in order to trigger this bug. If it's only 2 blocks high, slabs don't affect the spawn rate as expected.