Why are some artist photos missing in iTunes 12.7 list view

Why are some artist photos missing in iTunes 12.7 list view?

I have read elsewhere that the cause is that only music purchased from iTunes will have artists photos but that is not the case in my library as less than 25% of my 1200+ albums were purchased from iTunes store and a large number of imported CDs contain the photos.

I have also read that using iTunes Match will fill in the photo but that is not true as all of my imported CD's since iTunes 1.0 (mac), and they have been in iTunes Match since its inception.

Using the first hundred artists in my iTunes library:

of the artists with Icon, 26 were purchased and 38 were imported and

of the artists w/o icon, 7 were purchased and 29 were imported

In this case, of artists with photo icon, 41% were purchased but 59% were imported and

of those artists that had no icon, 7% were purchased and 81% were imported

Each album has the correct artwork.

This should be simple to manually change as when viewing by Artist in list view, selecting the artist name and then right-clicking and selecting 'Artist Info' it should bring up a dialog about the artist and allow adding a photo or other info but instead it brings up the same dialog as if you selected all tracks of that artist:

Is a known bug, is there any manual method of fixing this, is Apple aware of the problem?