With new wall behaviour being added in 1.16 that causes them to adapt better to the shape of surrounding blocks, stairs seem to have been left out, as they end up causing weird gaps if upside down stairs are positioned above walls. However, this may be considered a feature request.
One solution to this would be to have half of it raise up to perfectly match the stair area. However, as the inside area of walls are not textured, this may end up looking odd from some angles, and adding a texture to this usually hidden face would result in it being rendered in many cases where it wouldn't need to be (unless defined as a special element in the multipart that only appears for postless height mismatches), resulting in unneccessary processing power being spent on rendering a face that isn't even visible.
It may be more desirable to raise the half alongside having a post, which would occlude the missing wall face.
Hi, I can confirm in 1.16.4!