Intended behavior is that you can smooth a corner wall regardless of what its neighboring walls are like. Although unusual, there are times when one wants to have smooth corners and rough sides.
Since accessing corners has some special issues around it, we set it up so corners simply auto smooth when the adjacent walls are smoothed instead of requiring their own smoothing pass. They have no 'surface' after all. So it's not really a bug, just a quirk of the rules that were made to solve other issues.
Thanks for reporting either way.