In any game mode and in any type of world, you cannot place torches on the back of stairs or on the side of corner stairs. I'm reporting this as a bug as the back of stairs and side of corner stairs are flat, the same as normal blocks. Any questions just ask 🙂
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...please use the search function to see if your bug has already been submitted. Currently over 53% of tickets are being closed as duplicate.
search "torches stairs", first hit MC-8345. Duplicate.