How it still can be reproduced in 18w46a:
Place blocks in the following shape
Surround it with (full) blocks so that the lower slab is in full darkness
The top of that lower slab should now be dark but it is still bright
So I think this bug should be reopened
Not completely fixed (18w46a):
This also applies for random natural spawning (without a spawner). On grass on a mushroom island Mooshrooms do spawn - On mycel they do not. Definitely a bug.
How it still can be reproduced in 18w46a:
Place blocks in the following shape
Surround it with (full) blocks so that the lower slab is in full darkness
The top of that lower slab should now be dark but it is still bright
So I think this bug should be reopened
Not completely fixed (18w46a):
This also applies for random natural spawning (without a spawner). On grass on a mushroom island Mooshrooms do spawn - On mycel they do not. Definitely a bug.