All slabs and double slabs do glow in the dark (but don't emit any light) if following conditions are met:
no surrounding block- and skylight (lightlevel 0)
these slabs are somehow lit from the bottom side.
This bug is present since introduction of version 1.9!
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duplicates 1
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There are there to show, if any light is present, the slabs will behave normally. It only occures in total darkness.
We reopened the old MC-92 since it handled all inconsistencies with lighting and slabs/stairs back when it was even more broken.
So the (double) slabs are only visually lit.
I can confirm the double slab issue starts in 1.9, but I've tried the single slab issue is 1.8, and it was present there.
Also, what is the purpose of the 12.09.25 and 12.09.35 screenshots?