At night or in a dark room, if you look at the back side of stairs when there is a light source behind the stairs, there is a lighting quirk that occurs - most likely due to the multiple cuboids in the stairs model. Note that this issue only seems to occur with smooth lighting enabled.
This bug was previously mentioned in MCPE-8054, but since that ticket had become bloated with a total of 3 technically separate bugs (2 of which had been fixed), the remaining bug (this one) was reported separately and MCPE-8054 was resolved as fixed.
Steps to Reproduce:
Set your brightness to 0 and enable smooth lighting.
Go into a dark room or set the time to night.
Place down stairs. (quartz is the best for seeing the bug)
Place a torch in front of the stairs. (the side with the steps)
View the stairs from the flat back side.
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Fixed as of 1.16.101 by rendering stairs as transparent.

@unknown, I just tested in 1.16.100 and the issue is still there, though it's worth noting that it only occurs when smooth lighting is enabled (and is most clearly visible when brightness is set to 0). I've updated the issue description to mention this.
1.16.101 appears to be a Switch + mobile-only hotfix with no bugfixes related to lighting, so I'm guessing the bug isn't actually fixed in that version. If it does still appear to be fixed with smooth lighting enabled and brightness set to 0 (and make sure it's in a dark room or at night), let me know.

Still occurs on 1.16.221
instead of fixing the lights, they don't make lights pass through anymore, for java
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Still an issue in 1.20.62 and 1.20.70.24 Preview.

Thank you for your report!
After consideration, the issue is being closed as Won't Fix.
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