Slimes in swamp do not spawn on blocks at y=69

I constructed a mob farm in a swamp with the intention of collecting slime balls in addition to other mob drops.  The spawning platform was constructed at y = 69.  The AFK point is such that all mob spawning is constrained to the spawning platform, and the platform is covered in wither roses, so mobs are constantly dying and spawning at an extremely rapid rate. 

The spawning chamber is entirely within a swamp biome.  This behavior has been observed at all phases of the moon.  The spawning chamber has airspace of 2.5 blocks above the spawning platform.

I am using an item filter to capture any slime balls that emerge from the collection system, and no slime balls have been produced.  Many thousands of other item drops have been collected.

I have logged in with a secondary account set to spectator and observed and recorded video inside the spawning chamber.  No slimes ever spawn.

I tried removing the wither roses and replacing the netherrack with dirt, and slimes still did not spawn.

I tried removing the spawning platform and removing the rails from the collection layer underneath, at y = 67.  After doing this, I observed slimes spawning on this layer.

It appears that slimes spawn on blocks at y = 67, but not on blocks at y = 69.  All documentation I have read states that slimes in swamps spawn from y = 50 to y = 70.  It seems as if current spawning behavior does not follow the documentation.

Video demonstrating the issue here: https://youtu.be/25PfTwcqpco .

Center of spawning platform is at:

2187, 69, 3703

AFK position is:

2187, 195, 3703

Link to world:  https://drive.google.com/file/d/1rCWtDl_EbGNWOdwHwozf44-41t9UuYwD/view?usp=sharing

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