With Minecraft RTX coming, I believe that the "mechanics" for light SHOULD have parity as much as possible for both RTX and NON-RTX.
Therefore:
Slabs and Stairs SHOULD block light on vanilla.
I never understood why a roof made out of stairs or slabs lets through all light. As long as there are no holes, light shouldn't pass through, no matter RTX or NON-RTX. : )
Thank you for your report!
We're actually already tracking this issue in MCPE-363, so I've resolved and linked this ticket as a duplicate.
That ticket has already been resolved, which means this is not currently considered a bug. Please do not leave a comment on the linked ticket.
On a more personal note though, I believe the current expectation is that the new Render Dragon engine will be changing the way hidden lighting works. That's not official information and only time will tell if it turns out to be true 🙂
If you haven't already, you might like to make use of the search feature to see if the issue has already been mentioned.
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