I've noticed that the artists returned by sp_albumbrowse_similar_artist() do not have their portraits loaded (load status on sp_artist objects is true). By that, I mean that neither sp_artist_portrait() nor sp_link_create_from_artist_portrait() return valid results. When the exact same artists (verified by comparing URLs) are loaded by a search or through a playlist, they have portraits.
It looks to me as if it's perhaps an oversight, seeing how the Spotify client itself does not display portraits for similar artists. Or is there simply something I'm missing?