Flowing Lava and Water sometimes do not create cobblestone blocks when they meet

Sometimes whenever a flowing lava generates with a flowing water block above it, they do not create a cobblestone block. Oddly there are thin still water blocks beside it. 

 

Observed result: Flowing Lava and Water that are naturally generated above each other do not create a cobblestone block.

 

Expected result: They should combine to make a cobblestone block like normal. 

 

I've included the seed in the pictures to see if it can be replicated.