@chiark if an artist composes a song that then is performed by his current band, in later versions/releases when the artist performs the song solo or with another band, the credits for the artist/composer indeed should be "composer" and not "cover". Therefore, the settings for Paul McCartney are right
In my opinion, to reach what have been reached for Paul McCartney, you could give credits like this:
1. At the first release of the artist's own composed song, played with a band, you probably best could credit the composer-artist as the "artist" and the band as supporting artist and when you do that, the "work" (composition) will be added to that particular artist's "works". Because, when you mention the band as the artist and as an attribute give credits to the particular artist as composer, the song won't show up at the artist-composer's works, it will even not show up at his "relationships" only the band will show up, and then clicking on the band who performed it, the band is mentioned as the artist which is not right at all.
2. At a second release when the particular composer-artist goes solo or with another band, when having done what I said at 1. nothing odd will happen, the work stays credited as original work by that particular artist! and you only have to link the new recording to the original work!
I hope I could explain what I wanted to explain! It always is difficult explaining in English while not my native language, at least I hope this helps you!