Most of the half block blocks like slabs and hatches are translucent allowing the light to pass through them.
Linked issues
Comments 2
Thank you for your report!
We're actually already tracking this issue at MCPE-363, so I will resolve and link this ticket as a duplicate.
That ticket has already been resolved as working as intended, which means this is not considered a bug and won't be fixed. Please do not leave a comment on the linked ticket.
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That's how half slabs work